WASHINGTON, April 21 (Reuters) - President Joe Biden will deliver an update on the Russian invasion of Ukraine on Thursday as he works to complete a new arms package for its military.

Biden is to address Americans from the White House Roosevelt Room at 9.45 a.m. (1345 GMT) and a source familiar with the planning said he is "expected to provide an update on our efforts to support Ukraine and the assistance we are providing.”

The new arms package was likely to be roughly the same size as an $800 million one announced last week but details were still being worked out, another U.S. official 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.

If this week's aid package is as large as expected, it would bring total U.S. military aid to Ukraine since Russia invaded in to well over $3 billion.

After Biden's announcement, he is to fly to the Pacific Northwest on a two-day trip to promote his domestic agenda. (Reporting by Nandita Bose, Steve Holland, Patricia Zengerle and Idrees Ali Editing by Robert Birsel)