Ukraine closed its airspace to civilian flights on Thursday after Russia launched a "military operation" after weeks of intense diplomacy and the imposition of Western sanctions failed to deter Russian President Vladimir Putin.

Meanwhile, Europe's aviation regulator warned about hazards to flying in bordering regions. Moldova, southwest of Ukraine, also closed its airspace, while Belarus to the north said civilian flights could no longer fly over part of its territory.

The Foreign Office (FO) in Pakistan shared emergency contact details for its citizens in Ukraine to get in touch with the country's embassy for "timely assistance and guidance".

"Our embassy is available 24/7 to offer assistance to Pakistanis in Ukraine," the FO said in a tweet after Pakistan's embassy posted on its account that it was in contact with students who were unable to leave as per an earlier advice.

The embassy said the students had been asked to go to the Ternopil city, "where arrangements for their evacuation will be made as situation allows".

Putin announced a military operation in Ukraine early Thursday with explosions heard soon after in the capital and other parts of the country, prompting outrage from United States President Joe Biden who warned of a “catastrophic loss of life”.

Prior to the announcement, he had massed between 150,000 and 200,000 troops along the borders of Ukraine.

“I have made the decision of a military operation,” he said in a surprise statement on television shortly before 6am (0300 GMT).

Putin warned other countries that any attempt to interfere with the Russian action would lead to consequences they have never seen.

He accused the United States and its allies of ignoring Russia's demands to stop Ukraine from joining Nato and offer Moscow security guarantees. He said he wanted a “demilitarisation” of the former Soviet state but not its occupation.

Putin said all Ukrainian servicemen who lay down arms will be able to safely leave the zone of combat.

“All responsibility for bloodshed will be on the conscience of the ruling regime in Ukraine,” Putin said.

Ukraine airbases 'knocked out', says Russian military

The Russian military says it has knocked out Ukraine's air defence assets and airbases.

The Russian Defence Ministry said the Russian strikes have suppressed air defence means of the Ukrainian military, adding that the infrastructure of Ukraine's military bases has been incapacitated. It denied the claims that a Russian warplane was shot down over Ukraine.

The Ukrainian military, meanwhile, claimed to have shot down five Russian aircraft while fending off the "Russian attack on the country".

It later said that it destroyed four Russian tanks on a road near the eastern city of Kharkiv, killed 50 troops near a town in Luhansk region and downed a sixth Russian aircraft, also in the country's east.

Russia, however, has denied reports that its aircraft or armoured vehicles have been destroyed.

Ukraine's border guard service said that three of its servicemen had been killed in the southern Kherson region and that several more were wounded.

The Russian defence ministry earlier said it was targeting Ukrainian military infrastructure with precision weapons after President Vladimir Putin announced a military operation against the country.

“Military infrastructure, air defence facilities, military airfields, and aviation of the Armed Forces of Ukraine are being disabled with high-precision weapons,” the defence ministry said in a statement carried by Russian news agencies.

Ukraine imposes martial law

Ukraine's President Volodymyr Zelensky has declared martial law and said Russia was attacking his country's “military infrastructure”, but urged citizens not to panic and vowed victory.

His foreign minister said the worst-case scenario was playing out.

In a video message posted on Facebook after Russian President Vladimir Putin announced the launch of a military operation against Ukraine, Zelensky said he had spoken by phone with US President Joe Biden, saying "he vowed US support and assistance”.

'Act immediately'

Ukrainian Foreign Minister Dmytro Kuleba said Russia has launched a full-scale invasion of Ukraine and is targeting cities with weapons strikes.

"This is a war of aggression. Ukraine will defend itself and will win. The world can and must stop Putin. The time to act is now," Kuleba said in a tweet.

Shortly after Putin spoke, explosions could be heard in the pre-dawn quiet of the Ukrainian capital of Kyiv. Gunfire rattled near the capital's main airport, the Interfax news agency said.

Explosions also rocked the breakaway eastern Ukrainian city of Donetsk and civilian aircraft were warned away.

AFP correspondents also heard blasts in the Black Sea port city of Odessa.

Explosions also rang out in Kharkiv, a large city 35 kilometres south of the Russian border.

Four loud blasts rang out in Kramatorsk, a frontline city that serves as the Ukrainian government's effective capital for the eastern war zone, and more were heard in the eastern port city of Mariupol, AFP reporters said.

The scope of the Russian military operation was not immediately clear. Moscow has long denied that it has plans to invade despite massing tens of thousands of troops near its neighbour.

'Unprovoked, unjustified attack'

US President Joe Biden said his prayers were with the people of Ukraine “as they suffer an unprovoked and unjustified attack by Russian military forces”.

“President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering. Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way. The world will hold Russia accountable.”

He said he would announce further sanctions on Russia on Thursday, in addition to financial measures imposed this week.

The US hasn’t specified just what measures it will take now, although administration officials have made clear that all-out sanctions against Russia’s major banks are among the likely options. So are export limits that would deny Russia US high tech for its industries and military.

Another tough measure under consideration would effectively shut Russia out of much of the global financial system.

The Russian operations began as the United Nations Security Council held an emergency meeting on the Ukraine crisis in New York.

A draft resolution calling out Moscow over its actions towards its neighbour is doomed to fail due to Russia's veto power, however, a Security Council diplomat said it would put Russia on notice that it is “not in compliance with international law”.

Ukraine's president appeals to Russians

Putin's statement on Thursday came after the Kremlin said rebel leaders in eastern Ukraine had asked Moscow for military help against Kyiv.

In response, Ukraine's President Volodymyr Zelensky made an emotional late-night appeal to Russians not to support a “major war in Europe”.

Speaking Russian, Zelensky said that the people of Russia are being lied to about Ukraine and that the possibility of war also “depends on you”.

“Who can stop (the war)? People. These people are among you, I am sure,” he said.

Zelensky said he had tried to call Putin but there was “no answer, only silence”, adding that Moscow now had around 200,000 soldiers near Ukraine's borders.

Earlier the separatist leaders of Donetsk and Lugansk sent separate letters to Putin, asking him to “help them repel Ukraine's aggression”, Putin's spokesman Dmitry Peskov said.

The two letters were published by Russian state media and were both dated February 22.

Their appeals came after Putin recognised their independence and signed friendship treaties with them that include defence deals.

Tens of thousands of Russian troops are stationed near Ukraine's borders, and the West had said for days that an attack was imminent.

Living in fear

Western nations said ahead of Thursday's operation Russia had amassed 150,000 troops in combat formations on Ukraine's borders with Russia, Belarus and Russian-occupied Crimea and on warships in the Black Sea.

Ukraine has around 200,000 military personnel and could boost that with up to 250,000 reservists.

Moscow's total forces are much larger — around a million active-duty personnel — and have been modernised and re-armed in recent years.

But Ukraine has received advanced anti-tank weapons and some drones from Nato members.

More have been promised as the allies try to deter a Russian attack or at least make it costly.

Shelling had intensified in recent days between Ukrainian forces and Russia-backed separatists — a Ukrainian soldier was killed on Wednesday, the sixth in four days — and civilians living near the front were fearful.

Dmitry Maksimenko, a 27-year-old coal miner from government-held Krasnogorivka, told AFP that he was shocked when his wife came to tell him that Putin had recognised the two Russian-backed separatist enclaves.

“She said: 'Have you heard the news?'. How could I have known? There's no electricity, never mind internet. I don't know what is going to happen next, but to be honest, I'm afraid,” he said.

Russia has long demanded that Ukraine be forbidden from ever joining the Nato alliance and that US troops pull out from Eastern Europe.

How is Ukraine's economy holding up?

It was Ukraine, not Russia, where the economy was eroding the fastest under the threat of war.

One by one, embassies and international offices in Kyiv closed. Flight after flight was cancelled when insurance companies balked at covering planes arriving in Ukraine. Hundreds of millions of dollars in investment dried up within weeks.

The squeezing of Ukraine’s economy is a key destabilizing tactic in what the government describes as “hybrid warfare” intended to eat away at the country from within.

The economic woes include restaurants that dare not keep more than a few days of food on hand, stalled plans for a hydrogen production plant that could help wean Europe off Russian gas and uncertain conditions for shipping in the Black Sea, where container ships must carefully edge their way around Russian military vessels.