WASHINGTON -- The United States is committed to providing Ukraine with 鈥渢he weapons it needs鈥� to defend itself against Russia, U.S. national security adviser Jake Sullivan said on Sunday as Ukraine seeks more military aid from the West.

Sullivan said the Biden administration will send more weapons to Ukraine to prevent Russia from seizing more territory and targeting civilians, attacks that Washington has labeled war crimes.

鈥淲e鈥檙e going to get Ukraine the weapons it needs to beat back the Russians to stop them from taking more cities and towns where they commit these crimes,鈥� Sullivan said on ABC News鈥� 鈥淭his Week鈥�.

Moscow has rejected accusations of war crimes by Ukraine and Western countries.

Speaking later on NBC News鈥� 鈥淢eet the Press," Sullivan said the United States was 鈥渨orking around the clock to deliver our own weapons . . . and organizing and coordinating the delivery of weapons from many other countries.鈥�

鈥淲eapons are arriving every day,鈥� Sullivan said, 鈥渋ncluding today.鈥�

The United States has sent US$1.7 billion in military assistance to Ukraine since Russia launched its invasion on Feb. 24, the White House said last week.

Weapons shipments have included defensive anti-aircraft Stinger and anti-tank Javelin missiles, as well as ammunition and body armor. But U.S. and European leaders are being pressed by Ukraine President Volodymyr Zelenskiy to provide heavier arms and equipment to engage Russia in the eastern region of the country, where Russia is expected to intensify its military efforts. Read full story

On Friday, Ukrainian officials said more than 50 people were killed in a missile strike on a train station in city of Kramatorsk in the Donetsk region, where thousands of people had gathered to evacuate.

Russia's invasion has forced around a quarter of the population of 44 million to leave their homes, turned cities into rubble and killed or injured thousands.

Moscow has repeatedly denied targeting civilians in what it calls a "special operation" to demilitarize and "denazify" its southern neighbor. Ukraine and Western nations have dismissed this as a baseless pretext for war.

Russia on Saturday appointed a new general to lead its forces in Ukraine, Aleksandr Dvornikov, who had significant military experience in Syria.

With that background, Sullivan said he expects Dvornikov to authorize more brutality against the Ukrainian civilian population.

Republican U.S. Representative Liz Cheney, speaking on CNN鈥檚 鈥淪tate of the Nation," urged the Biden administration to provide Ukraine with both offensive weapons such as tanks and planes and defensive systems like anti-tank and anti-aircraft missiles.

"I think we need to do everything that Zelenskiy says he needs at this point, given the just unbelievable battle that they have put up," she said.

A CBS News poll released on Sunday showed widespread support among Americans for sending more weapons to Ukraine.

According to the poll, which was conducted last week as news of Russian attacks on civilians unfolded, 72 per cent of those surveyed favor sending more weapons, while 78 per cent support economic sanctions on Russia.