WASHINGTON, Sept 25 (Reuters) - The United States plans to announce more than $8 billion worth of military assistance for Ukraine on Thursday during Ukrainian President Volodymyr Zelenskiy's visit to Washington, two U.S. officials said on Wednesday. President Joe Biden's administration has been engaged in urgent discussions with Congress to allow it to use up $5.6 billion in military aid for Ukraine before Sept. 30 - the end of the federal fiscal year - when the authority was set to expire.Congress has approved nearly $175 billion of aid and military assistance for Ukraine and allied nations in the 2-1/2 years...