WASHINGTON, Aug 23 (Reuters) - The United States will announce a new security assistance package for Ukraine of about $3 billion as early as Wednesday, a U.S. official said on Tuesday, in what would be the single largest tranche to Kyiv since Russia's invasion six months ago.

The package is being prepared to coincide with Ukraine's independence day on Wednesday.

The package uses funds from the Ukraine Security Assistance Initiative (USAI) appropriated by Congress to allow the Biden administration to procure weapons from industry rather than taking weapons from existing U.S. weapons stocks.

Speaking on condition of anonymity, the official said the new weapons did not appear to include types of arms that had not been provided previously to the Ukrainian military. But the official said it would focus on ammunition and more medium-term objectives like defense systems.

Under the USAI, the weapons could take months to arrive in Europe given that companies have to procure them.

The official said the amount and mix of weapons could change before the formal announcement.

Since Russian troops invaded on Feb. 24 in what Russian President Vladimir Putin termed a "special military operation" to demilitarize Ukraine, the conflict has settled into a war of attrition fought primarily in eastern and southern Ukraine.

Washington has provided $10.6 billion in military assistance to President Volodymyr Zelenskiy's government since Feb. 24.

Germany plans to deliver further arms, including air-defense systems, rocket launchers and precision munitions, to Ukraine worth over 500 million euros ($500 million) in 2023, a source told Reuters.

Moscow is trying to gain control of the largely Russian-speaking Donbas region, comprised of Luhansk and Donetsk provinces, where pro-Moscow separatists seized territory after the Kremlin annexed Crimea to the south in 2014.

Ukraine accuses Moscow of an imperial-style war to retake a pro-Western neighbor that shook off Russian domination when the Soviet Union broke up in 1991.

In Ukraine, a sense of an eerie calm before the storm grew on Tuesday as the U.S Embassy told its citizens to leave Ukraine because of fears of possible Russian missile strikes as the country celebrates its 31 years of independence on Wednesday.

Kyiv has warned Moscow of a powerful response if it launches such strikes.