Russia pressed its invasion of Ukraine to the outskirts of the capital on Friday after unleashing airstrikes on cities and military bases and sending in troops and tanks from three sides in an attack that could rewrite the global post-Cold War security order.

Explosions sounded before dawn in Kyiv as Western leaders scheduled an emergency meeting and Ukraine’s president pleaded for international help.

Among the signs that the Ukrainian capital was increasingly threatened, the military said that a group of Russian spies and saboteurs was seen in a district of Kyiv about three miles north of the city centre.

Earlier, the military said that Russian forces had seized two Ukrainian military vehicles and some uniforms and were heading toward the city to try to infiltrate under the guise of being locals.

Ukraine President Volodymyr Zelenskyy said the government had information that “subversive groups” were encroaching on the city, and US Secretary of State Antony Blinken said Kyiv “could well be under siege” in what US officials believe is a brazen attempt by Russian President Vladimir Putin to dismantle the government and replace it with his own regime.

US Defence Secretary Lloyd Austin told lawmakers on a phone call on Thursday evening that Russian mechanised forces that entered from Belarus were about 20 miles from Kyiv, according to a person familiar with the call.

The assault, anticipated for weeks by the US and Western allies and undertaken by Putin in the face of international condemnation and cascading sanctions, amounts to the largest ground war in Europe since World War II.

Russian missiles bombarded cities and military bases on the first day of the attack, and Ukraine officials said they had lost control of the decommissioned Chernobyl nuclear power plant, the scene of the world’s worst nuclear disaster.

Civilians piled into trains and cars to flee and patrons of a hotel were directed into a shelter as explosions sounded in Kyiv.

“Russia has embarked on a path of evil, but Ukraine is defending itself and won’t give up its freedom,” Zelenskyy tweeted. His grasp on power increasingly tenuous, he called Thursday for even more severe sanctions than the ones imposed by Western allies and ordered a full military mobilisation that would last 90 days.

Ukraine's president vows to stay put

Zelenskyy said in a video address that 137 “heroes”, including 10 military officers, had been killed and 316 people wounded. The dead included border guards on the Zmiinyi Island in the Odesa region, which was taken over by Russians.

Zelenskiy vowed to stay in Kyiv as his troops battled Russian invaders advancing toward the capital.

“(The) enemy has marked me down as the number one target,” Zelenskiy warned in a video message as heavy fighting was reported on multiple fronts. “My family is the number two target. They want to destroy Ukraine politically by destroying the head of state.”

“I will stay in the capital. My family is also in Ukraine.”

He concluded an emotional speech by saying that “the fate of the country depends fully on our army, security forces, all of our defenders.” He also said the country had heard from Moscow that ”they want to talk about Ukraine’s neutral status.”

US imposes new sanctions

Biden was to meet on Friday morning with fellow leaders of Nato governments in what the White House described as an “extraordinary virtual summit” to discuss Ukraine.

US President Joe Biden announced new sanctions against Russia, saying Putin “chose this war” and had exhibited a “sinister” view of the world in which nations take what they want by force. Other nations also announced sanctions, or said they would shortly.

“It was always about naked aggression, about Putin’s desire for empire by any means necessary — by bullying Russia’s neighbours through coercion and corruption, by changing borders by force, and, ultimately, by choosing a war without a cause,” Biden said.

Read: Condemnations, warnings and sanctions: What global leaders are saying as Russia-Ukraine conflict intensifies

Blinken said in television interviews that he was convinced that Russia was intent on overthrowing the Ukrainian government, telling CBS that Putin wants to “reconstitute the Soviet empire” and that Kyiv was already “under threat, and it could well be under siege.”

The US sanctions will target Russian banks, oligarchs, state-controlled companies and high-tech sectors, Biden said, but they were designed not to disrupt global energy markets. Russian oil and natural gas exports are vital energy sources for Europe.

Zelenskyy urged the US and West to go further and cut the Russians from the SWIFT system, a key financial network that connects thousands of banks around the world. The White House has been reluctant to immediately cut Russia from SWIFT, worried it could cause enormous economic problems in Europe and elsewhere in the West.

Fearing a Russian attack on the capital city, thousands of people went deep underground as night fell, jamming Kyiv’s subway stations.

At times it felt almost cheerful. Families ate dinner. Children played. Adults chatted. People brought sleeping bags or dogs or crossword puzzles — anything to alleviate the waiting and the long night ahead.

But the exhaustion was clear on many faces. And the worries.

“Nobody believed that this war would start and that they would take Kyiv directly,” said Anton Mironov, waiting out the night in one of the old Soviet metro stations. “I feel mostly fatigue. None of it feels real.”

The invasion began early Thursday with a series of missile strikes, many on key government and military installations, quickly followed by a three-pronged ground assault.

Ukrainian and US officials said Russian forces were attacking from the east toward Kharkiv, Ukraine’s second-largest city; from the southern region of Crimea, which Russia annexed in 2014; and from Belarus to the north.

Zelenskyy, who had earlier cut diplomatic ties with Moscow and declared martial law, appealed to global leaders, saying that “if you don’t help us now, if you fail to offer a powerful assistance to Ukraine, tomorrow the war will knock on your door.”

Though Biden said he had no plans to speak with Putin, the Russian leader did have what the Kremlin described as a “serious and frank exchange” with French President Emmanuel Macron.

Both sides claimed to have destroyed some of the other’s aircraft and military hardware, though little of that could be confirmed.

Chernobyl takeover

Hours after the invasion began, Russian forces seized control of the now-unused Chernobyl plant and its surrounding exclusion zone after a fierce battle, presidential adviser Myhailo Podolyak told The Associated Press.

The Vienna-based International Atomic Energy Agency said it was told by Ukraine of the takeover, adding that there had been “no casualties or destruction at the industrial site.”

The 1986 disaster occurred when a nuclear reactor at the plant 80 miles north of Kyiv exploded, sending a radioactive cloud across Europe. The damaged reactor was later covered by a protective shell to prevent leaks.

Adviser to the commander of Ukraine’s ground forces, Alyona Shevtsova, wrote on Facebook that staff members at the Chernobyl plant had been “taken hostage”. The White House said it was “outraged” by reports of the detentions.

The Ukrainian Ministry of Defence issued an update saying that though the plant was “likely captured”, the country’s forces had halted Russia’s advance toward Chernihiv and that it was unlikely that Russia had achieved its planned Day One military objectives.

While some nervous Europeans speculated about a possible new world war, the US and its Nato partners have shown no indication they would send troops into Ukraine, fearing a larger conflict.

Nato reinforced its members in Eastern Europe as a precaution, and Biden said the US was deploying additional forces to Germany to bolster Nato.

European authorities declared the country’s airspace an active conflict zone.

After weeks of denying plans to invade, Putin launched the operation on a country the size of Texas that has increasingly tilted toward the democratic West and away from Moscow’s sway.

The autocratic leader made clear earlier this week that he sees no reason for Ukraine to exist, raising fears of possible broader conflict in the vast space that the Soviet Union once ruled. Putin denied plans to occupy Ukraine, but his ultimate goals remain hazy.

Ukrainians were urged to shelter in place and not to panic.

“Until the very last moment, I didn’t believe it would happen. I just pushed away these thoughts,” said a terrified Anna Dovnya in Kyiv, watching soldiers and police remove shrapnel from an exploded shell. “We have lost all faith.”

With social media amplifying a torrent of military claims and counter-claims, it was difficult to determine exactly what was happening on the ground.

Russia and Ukraine made competing claims about the damage they had inflicted. Russia’s Defence Ministry said it had destroyed scores of Ukrainian air bases, military facilities and drones.

It confirmed the loss of one of its Su-25 attack jets, blaming “pilot error,” and said an An-26 transport plane had crashed because of technical failure, killing the entire crew. It did not say how many were aboard.

Russia said it was not targeting cities, but journalists saw the destruction in many civilian areas.

Pakistan working on evacuating citizens

A number of countries including Pakistan moved to ensure the safety of their citizens in the war-ravaged country. Pakistan's ambassador to Ukraine, Dr Noel Israel Khokhar, said the government was working on the safe evacuation of all Pakistan nationals stranded in the country.

According to a report by Radio Pakistan, the ambassador confirmed that overall 1,500 Pakistanis, including 500 students, were present in Ukraine who had been asked to move to safer locations.

On the other hand, Pakistan's Embassy in Ukraine said in a tweet that "the airspace of Ukraine is closed and the Embassy is in touch with the Pakistani students, who could not leave earlier according to the advice given to them."