By Andrea Shalal

WASHINGTON (Reuters) - The United States has sent $3.4 billion in additional budget aid to Ukraine, giving the war-torn country critical resources amid intensifying Russian attacks on Ukrainian civilians and infrastructure, Treasury Secretary Janet Yellen said on Monday.

Yellen said in a statement the direct budget assistance, provided in coordination with the U.S. Agency for International Development and the State Department, marked the final disbursement under the 2024 Ukraine Security Supplemental Appropriations Act.

A U.S. official said the funding brings the total in U.S. budget aid to Ukraine to just over $30 billion since Russia's invasion in February 2022. Most of those funds are used to keep Ukraine's government running by paying salaries to teachers and other state employees. 

Earlier on Monday, U.S. President Joe Biden announced $2.5 billion in additional security assistance for Ukraine, which is separate from the direct budget aid.

Nearly three years into the war, Washington has committed $175 billion in total assistance for Ukraine.

President Joe Biden's administration has been racing to shore up support for Ukraine before Republican President-elect Donald Trump takes office on Jan. 20, given his public questioning of military aid and vows to end the Ukraine war within 24 hours of taking office.

Trump wants Ukraine's President Volodymyr Zelenskiy to make a deal with Russian President Vladimir Putin to end the nearly three-year-old Ukraine war. Some of his fellow Republicans, who will control both houses of the U.S. Congress starting next month, have also cooled on sending more aid to Kyiv.

Yellen said continued economic aid for Ukraine was crucial to allow it to maintain government services and continue to defend its sovereignty, warning against moves to cut funding.

"Ukraine’s success is in America’s core national interest," she said, vowing to continue to pressure Moscow with sanctions and to help position Ukraine to achieve a just peace. 

"We must not retreat in this effort," she said.

She stressed that U.S. budget aid for Ukraine has been and remains conditioned on reforms aimed at strengthening law enforcement, improving the transparency and efficiency of government institutions, and bolstering anti-corruption efforts.

The latest funding came on top of the $20 billion U.S. portion of a $50 billion Group of Seven loan for Ukraine that Treasury transferred to a World Bank intermediary fund for Ukraine earlier this month. Those funds are backed by profits earned on frozen Russian sovereign assets.

Biden on Wednesday said he had asked the Defense Department to continue its surge of weapons deliveries to Ukraine, after condemning Russia's Christmas Day attack on Ukraine's energy system and some of its cities.  

(Reporting by Andrea Shalal; Editing by Don Durfee and Chris Reese)