WASHINGTON- U.S. President Joe Biden announced an additional $800 million security assistance package for Ukraine on Thursday morning to further shore up support for the embattled European country as it faces a fresh onslaught by Russia on its eastern flank.

Addressing Americans from the White House's Roosevelt Room, Biden pledged to send dozens of howitzers, 144,000 rounds of ammunition and tactical drones, as he called on Congress for supplemental funding to provide additional aid for Kyiv.

"We’re in a critical window now of time where they’re going to set the stage for the next phase of this war,” Biden said. The United States and allies are “moving as fast as possible” to provide Ukraine with the equipment and weapons it needs.

The president also announced plans to ban Russian-affiliated ships from U.S. borders, as previously reported by Reuters, further ratcheting up pressure on Moscow.

The new arms package is the same size as an $800 million one announced last week but details are still being worked out, a U.S. official, speaking on condition of anonymity, told Reuters earlier.

On Wednesday, Biden convened U.S. military leaders in an annual White House gathering that took on special significance as the war enters a risky new phase.

Opening the meeting, Biden touted the toughness of the Ukrainian military and said that NATO's unity had shocked Russian President Vladimir Putin.

Russia has said it has entered a new stage of its operation and is methodically seeking to "liberate" the Donbas region of eastern Ukraine. Western allies anticipate Russia's campaign could last many months, grind to a stalemate and test the battlefield capabilities of Ukrainian fighters.

Russia says it launched what it calls a "special military operation" on Feb. 24 to demilitarize and "denazify" Ukraine. Kyiv and its Western allies reject that as a false pretext.

U.S. forces are not fighting in Ukraine but are indirectly engaged, arming, training and financing its forces.

The U.S. aid announced last week included artillery systems, artillery rounds, armored personnel carriers and unmanned coastal defense boats, broadening the scope of materiel sent to Kyiv to include new types of heavy equipment.

Adding this week's package brings total U.S. military aid to Ukraine since Russia invaded to well over $3 billion.

Following his announcement, Biden will fly to the Pacific Northwest for a two-day trip to promote his domestic agenda.

(Reporting by Nandita Bose, Steve Holland, Patricia Zengerle and Idrees Ali; Editing by Robert Birsel, Chizu Nomiyama and Jonathan Oatis)