Russia said its forces had stopped firing near two Ukrainian cities on Saturday to allow safe passage to civilians fleeing fighting, but was continuing its broad offensive in Ukraine, where the capital Kyiv came under renewed assault.

The Russian defence ministry said its units had opened humanitarian corridors near the cities of Mariupol and Volnovakha which were encircled by its troops, Russia's RIA news agency reported.

In Mariupol, citizens would be allowed to leave during a five-hour window, it quoted the city's officials as saying.

The Ukrainian government said the plan was to evacuate around 200,000 people from Mariupol and 15,000 from Volnovakha, and the Red Cross is the ceasefire's guarantor.

There was no immediate confirmation that firing had stopped and it was not clear if the ceasefire would be extended to other areas, as Russia's invasion of Ukraine entered into its 10th day.

The Russian defence ministry said a broad offensive would continue in Ukraine, RIA said.

Aid agencies have warned of an unfolding humanitarian disaster as food, water and medical supplies run short and refugees stream into western Ukraine and neighbouring European countries.

In the southeastern port city of Mariupol - whose capture would be a key prize for Russia - there is no water, heat or electricity and food is running out, according to Mayor Vadym Boychenko.

“We are simply being destroyed,” he said.

Ukraine 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, was again under attack, with explosions audible from the city centre.

Ukrainian media outlet Suspilne cited authorities in Sumy, about 300 km (190 miles) east of Kyiv, as saying that there is a risk of fighting in the city's streets, urging residents to stay in shelters.

A Ukrainian negotiator, Mykhailo Podolyak, had said on Thursday that a second round of ceasefire talks with Russia had not yielded the results Kyiv hoped for, but both sides had reached an understanding on creating humanitarian corridors.

Defence Minister Oleksii Reznikov said 66,224 Ukrainian men had returned from abroad to join the fight against Russia's invasion. "These are 12 more combat and motivated brigades! Ukrainians, we are invincible," Reznikov said in an online post.

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.

Moscow denies targeting civilians in Ukraine and says its aim is to disarm its neighbour, counter what it views as NATO aggression and capture leaders it calls neo-Nazis.

Plea for more help

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.

Ukrainian President Volodymyr Zelenskiy was expected to press Washington for more help in a video call with the US Senate at 9:30 a.m. 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% this week on concern about supply shortages, posing a risk to global economic growth.

No to NO-FLY Zones

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.

Zelenskiy slammed the summit, saying "It was clear that not everyone considers the battle for Europe's freedom to be the number one goal".

Ukraine's military said in a statement on Saturday that armed forces "are fighting fiercely to liberate Ukrainian cities from Russian occupiers," counterattacking in some areas and disrupting communications.

"Units of the invaders are demoralized, soldiers and officers of the occupying army continue to surrender, flee, leaving weapons and equipment on Ukrainian soil," it said, adding that at least 39 Russian planes and 40 helicopters had been destroyed. Reuters has not been able to independently verify such accounts.

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.”

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.