Chairman of the Joint Chiefs of Staff Gen. Mark Milley told lawmakers that Kyiv could fall within 72 hours if a full-scale Russian invasion of Ukraine takes place, multiple congressional sources tell Fox News.

Ukrainian President Volodymyr Zelenskyy answers questions from the media in Kyiv, Ukraine, on Friday.

Milley told lawmakers during closed-door briefings on Feb. 2 and 3 that a full-scale Russian invasion of Ukraine could result in the fall of Kyiv within 72-hours, and could come at a cost of 15,000 Ukrainian troop deaths and 4,000 Russian troop deaths.

Several lawmakers expressed concern that the Biden administration did not respond quickly to provide Ukraine with significant military aid, such as anti-aircraft and rocket launcher systems that would defend against an invasion from Russia.

Biden administration officials at the meetings responded to these concerns by saying that a significant supply of military aid to Ukraine could be used as a reason to invade Ukraine. While Ukraine is receiving security assistance from the United States through a $200 million allocation made in December,

FILE – A Russian tank T-72B3 fires as troops take part in drills at the Kadamovskiy firing range in the Rostov region in southern Russia, Wednesday, Jan. 12, 2022. 

it is also receiving $600 million in assistance from the United Kingdom and Baltic states, which the country is depending heavily upon and includes anti-tank Javelin missiles and older-version Stinger portable anti-aircraft missiles.

Several Biden administration officials were present at the briefings, including Secretary of State Anthony Blinken, Secretary of Defense Lloyd Austin, Director of National Intelligence Avril Haines, Chairman of the Joint Chiefs of Staff Gen. Mark Milley, Homeland Security Secretary Alejandro Mayorkas, Deputy Treasury Secretary Wally Adeyemo, Deputy Commerce Secretary Don Graves, and USAID Deputy Administrator Ambassador Isobel Coleman.

Russian President Vladimir Putin gestures while speaking to the media during a joint news conference with Hungary’s Prime Minister Viktor Orban following their talks in the Kremlin in Moscow, Russia, Tuesday, Feb. 1, 2022. 

The United States is also working to make preparations for a significant refugee and migrant crisis, one person familiar with the planning said. European allies are also leading discussions on housing and migrant flows under the assumption that Ukrainian nationals would flee to other portions of Europe.