WASHINGTON, March 21 (Reuters) - The United States is authorising another US$350 million in military aid for Ukraine, US Secretary of State Antony Blinken said on Monday (Mar 20), as Kyiv builds up its arsenal for an anticipated counter-offensive against Russian forces.

"This military assistance package includes more ammunition for US-provided HIMARS and howitzers that Ukraine is using to defend itself, as well as ammunition for Bradley Infantry Fighting Vehicles, HARM missiles, anti-tank weapons, riverine boats, and other equipment," Blinken said in a statement.

The United States has provided more than US$30 billion in weaponry to Ukraine to defend itself against Russia, which invaded its pro-Western neighbour on Feb 24, 2022.

The latest US assistance was announced days after US Defense Secretary Lloyd Austin, White House national security adviser Jake Sullivan and Chairman of the Joint Chiefs of Staff Mark Milley held a call with their Ukrainian counterparts and spoke of their "unwavering support" for Ukraine.

Ukrainian President Volodymyr Zelenskiyy had also joined the end of that call on Friday, the White House said.

Kyiv is seeking to gather sufficient supplies of arms from its Western backers, of which the US has been the most significant, to mount a counter-offensive and try to take back territory captured by the Russians last year.

"Russia alone could end its war today. Until Russia does, we will stand united with Ukraine for as long as it takes," Blinken said on Monday.

The announcement of the latest assistance package from Washington was made the same day that Chinese President Xi Jinping met Russian President Vladimir Putin in Moscow to promote Beijing's role as a potential peacemaker in Ukraine.

Xi was the first leader to meet Putin since the International Criminal Court (ICC) issued an arrest warrant for the Russian leader on Friday over the deportation of Ukrainian children to Russia.