Russian forces inched towards Kyiv on Saturday and pounded civilian areas in other Ukrainian cities as concerns grew over the besieged southern port of Mariupol, where officials said more than 1,500 people had been killed.

Air raid sirens sounded on Saturday in several cities, including the capital Kyiv, Odessa, Dnipro and Kharkiv, according to Ukrainian media.

More than two weeks after Moscow shocked the world by invading Ukraine, the United Nations and others said it may be committing war crimes in cities such as Mariupol, which for days has been under attack by Vladimir Putin's forces.

Survivors have been trying to flee Russian bombardment in a freezing city left without water or heating and running out of food. The situation is “desperate”, a Doctors Without Borders official said.

“Hundreds of thousands of people ... are for all intents and purposes besieged,” Stephen Cornish, one of those heading the medical charity's Ukraine operation, told AFP in an interview.

Ceasefire hopes

Several humanitarian corridors out of Ukrainian towns and villages including from the besieged southern port of Mariupol will be open on Saturday so civilians can leave, Ukraine's Deputy Prime Minister Iryna Vereshchuk said, adding she hoped Russia would observe a ceasefire to allow this to take place.

She said Ukraine plans to evacuate residents of several towns and villages in the regions of Kyiv and Sumy and some other areas where there is ongoing combat.

“I hope that the day will go well, all the planned routes will be open and Russia will fulfill its obligations to guarantee the ceasefire regime,” Vereshchuk said in a video address.

Efforts to provide safe passage for residents of Mariupol have repeatedly failed this week.

Ukraine at 'turning point'

Ukraine was at a turning point in the war with Russian forces appearing to be regrouping for a possible assault on Kyiv, President Volodymyr Zelensky said, as the United States imposed new sanctions on senior Kremlin officials and Russian oligarchs.

With the Russian assault in its third week, Zelensky, who has rallied his people with a series of addresses from the capital Kyiv, said Ukraine had "already reached a strategic turning point".

"It is impossible to say how many days we still have [ahead of us] to free Ukrainian land. But we can say we will do it," he said. "We are already moving towards our goal, our victory."

The United States on Friday imposed sanctions on Russian billionaire Viktor Vekselberg, three family members of President Vladimir Putin's spokesperson and lawmakers in the latest punishment for Russia's February 24 invasion.

"Treasury continues to hold Russian officials to account for enabling Putin's unjustified and unprovoked war," Treasury Secretary Janet Yellen said.

The fighting has created more than two million refugees, and thousands of Ukrainians are trapped in besieged cities.

As hundreds sheltered in Kharkiv metro stations, Nastya, a young girl lying on a makeshift bed on the floor of a train carriage, said she had been there for over a week, unable to move around much and ill with a virus.

"I'm scared for my home, for the homes of my friends, very scared for the whole country, and scared for myself of course," she said.

Putin calls the invasion a "special operation" to disarm Ukraine and unseat leaders it calls neo-Nazis. Ukraine and Western allies call this a baseless pretext for a war of choice that has raised fears of wider conflict in Europe.

Psychiatric hospital

The governor of the Kharkiv region on the Russian border said a psychiatric hospital had been hit, and the mayor of the city of Kharkiv said about 50 schools there had been destroyed.

In the besieged southern city of Mariupol, the city council said at least 1,582 civilians had been killed by Russian shelling and a 12-day blockade that has left hundreds of thousands trapped with no food, water, heat or power.

Moscow denies targeting civilians.

Russia's defence ministry said the Black Sea port was surrounded, while Ukrainian officials accused Russia of deliberately preventing civilians from getting out and humanitarian convoys from getting in.

A new effort to evacuate civilians along a humanitarian corridor from Mariupol appeared to have failed, as Ukrainian Deputy Prime Minister Iryna Vereshchuk said Russian shelling prevented them from leaving.

"The situation is critical," Ukrainian interior ministry adviser Vadym Denysenko said.

Western countries meanwhile took more economic steps to try to force Putin to end his assault.

President Joe Biden, who this week banned US imports of Russian oil, said the G7 industrial powers would revoke Russia's "most favoured nation" trade status.

Biden banned US imports of Russian seafood, alcohol and diamonds. Washington sanctioned more oligarchs and elites, including board members of Russian banks, and a dozen lawmakers.

European Union (EU) leaders said they were ready to impose harsher sanctions on Russia and might give Ukraine more funds for arms. But they rejected Ukraine's request to join the bloc.

European Commission President Ursula von der Leyen said that the EU would on Saturday suspend Moscow's privileged trade and economic treatment, crack down on its use of crypto-assets, and ban the import of iron and steel goods from Russia, as well as the export of luxury goods in the other direction.

Gearing up

Russia's main force has been stalled north of Kyiv, having failed in what Western analysts say was an initial plan for a lightning assault.

But Britain's defence ministry said Russia appeared to be gearing up for a new offensive in the coming days that would probably include Kyiv.

Images taken on Friday and released by private US satellite firm Maxar showed Russian forces were continuing to deploy closer to Kyiv and firing artillery toward residential areas, according to the company's analysis.

Multiple homes and buildings were on fire and widespread damage was seen throughout the town of Moschun, northwest of Kyiv, Maxar said. Reuters could not independently verify the images.

But Britain's intelligence update said Russian ground forces were still making only limited progress, hampered by logistical problems and Ukrainian resistance.

The Ukrainian general staff said Russian forces were regrouping after taking heavy losses. Ukrainian troops had pushed some back to "unfavourable positions" near the Belarus border, it said.

Kyiv's mayor, former heavyweight boxing champion Vitali Klitschko, said the capital had enough essential supplies to last a couple of weeks. Supply lines remained open.

Belarus

At a meeting with Belarus President Alexander Lukashenko, Putin said there were "certain positive shifts" in talks with Kyiv, but did not elaborate.

Ukraine has raised the prospect of Moscow's ally Belarus entering the war, accusing Russia of staging "false flag" air attacks on Belarus from Ukraine to provide an excuse.

Belarus has served as a staging post for Russian forces before and after the invasion. The Kremlin did not respond to a request for comment.

Putin and Lukashenko agreed Moscow would supply its smaller neighbour with up-to-date military equipment, the official Belarus Belta news agency said.

Moscow might not have sufficient troops to achieve its goals, said Mathieu Boulegue, an expert at London's Chatham House think tank.

"You can't invade a country on a one-on-one ratio" of troops, Boulegue told Reuters. "Nobody has done it, which means that either something was wrong or they had very wrong assumptions."