WASHINGTON, Aug 18 (Reuters) - President Joe Biden's administration is readying about $800 million of additional military aid to Ukraine and could announce it as soon as Friday, three sources familiar with the matter said on Thursday.

Biden would authorize the assistance using his Presidential Drawdown Authority, which allows the president to authorize the transfer of excess weapons from U.S. stocks, the sources told Reuters.

The sources, speaking on condition of anonymity, said that an announcement could slip into next week, cautioning that weapons packages can change in value before they are announced.

The White House declined to comment.

Since Russian troops poured over the Ukrainian border in February in what Russian President Vladimir Putin termed a "special military operation," the conflict has settled into a war of attrition fought primarily in the east and south of Ukraine.

Washington has sent billions of dollars in security assistance to the Kyiv government.