Ukrainian servicemen at a military camp near Lviv. File/AFP

Russian air strikes killed 35 people at a military base outside Ukraine's western city of Lviv, the regional governor said Sunday, updating the toll from nine previously.

"I have to announced that, unfortunately, we have lost more heroes: 35 people died as a result of the shelling of the International Peacekeeping and Security Centre," the governor of Lviv region Maxim Kozitsky wrote on Telegram.

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Russian forces shelled mosque in Mariupol

Air raid sirens across Ukraine as Russian forces regroup for possible assault on Kyiv

"The occupiers launched an air strike on the International Center for Peacekeeping and Security. According to preliminary data, they fired eight missiles," the administration said in a statement.

The centre, less than 25 km (15 miles) from the Polish border, did not say whether it had been hit. It said it would release details later.

Meanwhile, a relentless assault on the Ukrainian port city of Mariupol continued on Saturday, as Russian forces shelled the city's downtown, including an area around a mosque that was sheltering more than 80 people — some children.

Russian Deputy Foreign Minister Sergei Ryabkov said Moscow has warned the United States that Moscow could attack convoys carrying military equipment to Ukraine, calling them "legitimate targets.” US President Joe Biden announced additional aid to Ukraine of up to $200 million for weapons, military services, education and training.

Ukrainian President Volodymyr Zelenskiy speaks in Kyiv, Ukraine. File/AP

Earlier, Ukrainian President Volodymyr Zelensky has warned Russian forces they face a fight to the death if they try to occupy the capital Kyiv, as air raid sirens again woke residents on Sunday morning.

"If they decide to carpet bomb and simply erase the history of this region ... and destroy all of us, then they will enter Kyiv. If that's their goal, let them come in, but they will have to live on this land by themselves," Zelenskiy said on Saturday.

Zelenskiy on Saturday accused Russia of trying to create new "pseudo-republics” to break his country apart. He called on Ukraine’s regions not to repeat the experience of two eastern regions where pro-Russian separatists began fighting Ukrainian forces in 2014.

US President Joe Biden speaks during a meeting.

Russian units fanned out to prepare for an assault on Ukraine's capital of Kyiv. Zelenskyy said Russia would need to carpet-bomb Kyiv and kill its residents to take the city.

Now in its third week, the war has forced more than 2.5 million people to flee Ukraine.

Ukrainian President, who has repeatedly appeared on social media from the capital, said some small towns no longer existed in the third week of Russian attacks, the biggest assault on a European country since World War Two.

Russian shelling has trapped thousands of people in besieged cities and sent 2.5 million Ukrainians fleeing to neighbouring countries.

Ukraine accused Russian forces on Saturday of killing seven civilians in an attack on women and children trying to flee fighting near Kyiv. France said Russian President Vladimir Putin had shown no readiness to make peace.