US President Joe Biden has marked Ukraine's Independence Day with a new security assistance package of about $US3 billion ($A4.3 billion) in military aid for Kyiv in its war with Russia. It is the single largest tranche of support for Kyiv since Russia's invasion six months ago. It comes as US officials warn that Russia appears to be planning fresh attacks against Ukraine's civilian infrastructure and government facilities in coming days. "The United States of America is committed to supporting the people of Ukraine as they continue the fight to defend their sovereignty," Biden said in a statement announcing the package on Wednesday. In total, the United States has committed approximately $US10.6 billion ($A15.3 billion) in security assistance to Ukraine since the beginning of the Biden Administration. Since 2014, the United States has committed more than $US12.6 billion ($A18.2 billion) in security assistance to Ukraine. Australian Associated Press