Biden also told Zelensky that the United States would provide the Ukrainian government with $500 million in direct aid, as Kyiv battles against invading Russian forces.

"The leaders discussed... continued efforts by the United States with allies and partners to identify additional capabilities to help the Ukrainian military defend its country," the White House said.

Some 20,000 people are believed to have been killed since the Russian invasion began on February 24, according to Zelensky, though the number of casualties could not be independently verified.

"Just finished an hour-long conversation," Zelensky tweeted after the call. "Shared assessment of the situation on the battlefield and at the negotiating table.

"Talked about specific defensive support, a new package of enhanced sanctions, macro-financial and humanitarian aid."

Ukraine and Western powers have cast doubt on Russia's pledge to reduce military activity around Chernigiv and the capital Kyiv, made during face-to-face talks in Istanbul on Tuesday.

Pentagon officials said not all the weaponry Biden promised in mid-March, in particular kamikaze-like drones called Switchblades or Soviet-made S-300 anti-aircraft batteries, has been delivered yet.

"We have committed 100 Switchblade tactical unmanned aerial systems to be delivered in the most recent package of presidential drawdown," Celeste Wallander, assistant secretary of defense for International Security Affairs, told a congressional hearing Wednesday.

"Those are in the package that is in process of being delivered," Wallander said.

On the S-300s she said "we have focused on getting countries that hold Soviet legacy systems including S-300 systems, that have spare parts, missiles, different parts of that S-300 system, who are willing to send that to Ukraine."

"That is ongoing," Wallander said.

The United States is in discussions with Slovakia, which wants to replace its S-300s with more modern US-made Patriot missile batteries, she said.

The United States has "two centers" with "approximately 100 individuals" that are working to get military assistance and humanitarian assistance to Ukraine, General Tod Wolters, the commander of US forces in Europe, told lawmakers.