Russia declared a partial ceasefire on Saturday to allow humanitarian corridors out of the Ukrainian cities of Mariupol and Volnovakha, Russia's defence ministry said.

“From 10am Moscow time (0700 GMT), the Russian side declares a ceasefire and the opening of humanitarian corridors to allow civilians to leave Mariupol and Volnovakha,” Russian news agencies quoted the Russian defence ministry as saying.

Mariupol, a southern city of about 450,000 people on the Azov Sea, will begin evacuations at 0900 GMT, city hall announced on social media in a message that added, “it will be possible to leave the city by private transport.”

“A huge request to all drivers leaving the city, to contribute as much as possible to the evacuation of the civilian population — take people with you, fill vehicles as much as possible,” the statement said.

The announcement said the evacuation would last over several days to allow the entirety of the civilian population to exit the city.

In the statement, city officials told residents leaving in private vehicles that it was “strictly prohibited” to go off course from the evacuation routes.

Municipal buses were also departing from three locations in the city to help people leave, the message said.

Ukraine Deputy Prime Minister Iryna Vereshchuk wrote on social media that some 200,000 people were expected to be extracted from the city.

She wrote that a further 15,000 people would be brought from Volnovakha, a town of around 20,000 people some 60 kilometres from separatist-controlled Donetsk, a regional centre.

“This is not an easy decision, but, as I have always said, Mariupol is not its streets or houses. Mariupol is its population, it is you and me,” mayor Vadim Boychenko was quoted as saying in the statement.

With Russian troops surrounding the city, he said, “there is no other option but to allow residents — that is, you and me — to leave Mariupol safely,” he said.

Russia blocks Facebook, other sites

Meanwhile, Russia blocked Facebook and some other websites and passed a law that gave Moscow much stronger powers to crack down on journalism, prompting the BBC, Bloomberg and other foreign media to suspend reporting in the country.

War raged in Ukraine for a 10th day on Saturday as Russian troops besieged and bombarded cities.

The fighting has created over one million refugees, a barrage of sanctions that are increasingly isolating Moscow and fears in the West of a wider global conflict that has been unthought-of for decades.

Moscow says its invasion is a “special operation” to capture individuals it regards as dangerous nationalists, and has denied targeting civilians.

Ukraine's state service of special communications and protection of information says Russian forces have focussed efforts on encircling Kyiv and Kharkiv, the second-biggest city, while aiming to establish a land bridge to Crimea.

Kyiv, in the path of a Russian armoured column that has been stalled outside the Ukrainian capital for days, came under renewed assault, with explosions audible from the city centre.

Ukrainian media outlet Suspilne cited authorities in Sumy, about 300 kilometres east of Kyiv, as saying that there is a risk of fighting in the city's streets, urging residents to stay in shelters.

Russian forces also have encircled and shelled the southeastern port city of Mariupol — a key prize. There is no water, heat or electricity and food is running out, according to Mayor Vadym Boychenko.

“We are simply being destroyed,” he said.

President Vladimir Putin's actions have drawn almost universal condemnation, and many countries have imposed heavy sanctions as the West balances punishment with avoiding a widening of the conflict.

Fighting back in the information war, Russia's parliament passed a law on Friday imposing a prison term of up to 15 years for spreading intentionally “fake” news about the military.

“This law will force punishment — and very tough punishment — on those who lied and made statements which discredited our armed forces,” said Vyacheslav Volodin, the chairman of the Duma, Russia's lower house of parliament.

Russia is blocking Facebook for restricting state-backed channels and the websites of the BBC, Deutsche Welle and Voice of America.

CNN and CBS News said they would stop broadcasting in Russia, and other outlets removed Russian-based journalists' bylines as they assessed the situation.

More sanctions on the way?

Ukrainian President Volodymyr Zelensky is expected to press Washington for more help in a Zoom call with the full United States Senate at 9:30am ET (1430 GMT) on Saturday.

The United States is weighing cuts to imports of Russian oil and ways to minimise the impact on global supplies and consumers as lawmakers fast-track a bill that would ban Russian energy imports. Global oil prices surged over 20 per cent this week on fears of supply shortages, posing a risk to global economic growth.

At a meeting on Friday, Nato allies rejected Ukraine's appeal for no-fly zones, saying they were increasing support but that stepping in directly could make the situation worse.

“We have a responsibility ... to prevent this war from escalating beyond Ukraine because that would be even more dangerous, more devastating and would cause even more human suffering,” said Nato Secretary-General Jens Stoltenberg.

Zelensky slammed the summit as “weak” and “confused”. “It was clear that not everyone considers the battle for Europe's freedom to be the number one goal,” he said.

More EU sanctions were coming, potentially including a ban on Russian-flagged ships in European ports and blocking imports of steel, timber, aluminium or coal, said Irish Foreign Minister Simon Coveney.

Russian Defence Minister Sergei Shoigu told UN Secretary General Antonio Guterres on Friday that talks with Ukraine on peacefully ending the conflict had “not moved from the starting point”, Tass news agency said.

Humanitarian disaster

A humanitarian disaster is unfolding, with more than one million people seeking refuge in western Ukraine and in neighbouring countries.

Thousands of people waited for hours on Friday outside the railway station at the western city of Lviv to board trains heading to Poland. Families arrived with few belongings. Some were in wheelchairs, others accompanied by pet dogs and cats, uncertain about their fate.

“All we took with us is the bare necessities,” said Yana Tebyakina. “A change of clothes. That's it. All the rest we left behind, all our lives stayed back at home.”

A Friday attack on the Zaporizhzhia nuclear power plant, the largest in Europe, about 230 km west of Mariupol, brought the conflict to a perilous moment, but officials later said the facility was safe.

The plant and adjacent territory were now being guarded by Russian troops, Moscow's envoy to the United Nations said.

The US ambassador to the UN, Linda Thomas-Greenfield, said the world had narrowly averted a nuclear catastrophe.

The attack reflected a “dangerous new escalation” in Russia's invasion, she said during an emergency UN Security Council meeting, demanding assurances from Moscow that such an assault would not happen again.

Russian forces have made their biggest advances in the south, where they captured their first sizeable Ukrainian city, Kherson, this week. Bombing has worsened in recent days in the northeast cities of Kharkiv and Chernihiv.

Ukrainian presidential adviser Oleksiy Arestovych said an advance had been halted on the southern port of Mykolayiv. If captured, the city of 500,000 people would be the biggest yet to fall.

Additional input from AFP.