US President Joe Biden has for the first time publicly apologised to Ukraine for a months-long congressional hold-up in American military assistance that let Russia make gains on the battlefield, as Ukrainian President Volodymyr Zelenskiy appealed for bipartisan US support “like it was during World War II”.

Speaking in Paris, a day after they both attended ceremonies marking the 80th anniversary of the D-Day landings in Normandy, Biden apologised to the Ukrainian people for the weeks of not knowing if more assistance would come while conservative Republicans in Congress held up a $US61 billion ($A92 billion) military aid package for Ukraine for six months.

Still, the Democratic President insisted that the American people were standing by Ukraine for the long haul. “We’re still in. Completely. Thoroughly,” he said.

The apology – and Zelenskiy’s plea for rock-solid support akin to the allied coalition in World War II – served as a reminder that for all of Biden’s talk of an unflagging US commitment to Ukraine, recalcitrance among congressional Republicans and an isolationist strain in American politics has led to Russian gains on the battlefield.

And, although unremarked upon, the spectre of Donald Trump’s candidacy loomed over the discussion, as the Republican former president and the presumptive nominee has spoken positively of Russian President Vladimir Putin and sparked Ukrainian concerns that he would call for it to cede territory to end the conflict.

Zelenskiy pressed for all Americans to support his country’s defence against Russia’s invasion, and he thanked lawmakers for eventually coming together to approve the weapons package, which has allowed Ukraine to stem Russian advances in recent weeks.

“It’s very important that in this unity, United States of America, all American people stay with Ukraine like it was during World War II,” Zelenskiy said.

“How the United States helped to save human lives, to save Europe. And we count on your continuing support in standing with us shoulder to shoulder.”

The United States is by far Kyiv’s biggest supplier of wartime support, and Ukraine is trying to fend off an intense Russian offensive in eastern areas of the country. The push is focused on the Ukrainian border regions of Kharkiv and Donetsk, but Ukrainian officials say it could spread as Russia’s bigger army seeks to make its advantage tell.

The offensive is seeking to exploit Kyiv’s shortages of ammunition and troops along the roughly 1000km front line.

–AAP