The United States is discussing the deployment of American military forces to Eastern Europe with its NATO allies, a senior administration official said Monday, as President Joe Biden weighs options for responding to Russian threats against Ukraine.

Defense Secretary Lloyd Austin has placed 8,500 troops on “heightened alert” to assist with the defense of NATO allies, Pentagon spokesman John Kirby said during a news briefing Monday. He said no deployment orders have been given.

"I want to reinforce that as of now, the decision has been made to put these units on higher alert and higher alert only," said Kirby. "No decisions have been made to deploy forces from the United States at this time."

With Moscow massing more than 100,000 troops at its neighbor's border and no diplomatic breakthrough in sight, the West is stepping up its response amid mounting fears an invasion could be imminent.

Conversations are underway with NATO countries that could receive U.S. military forces as part of a plan to deter Russian President Vladimir Putin’s aggression, the official said.

NATO said Monday that it was sending ships and fighter jets to Eastern Europe and that Washington “has also made clear that it is considering increasing its military presence in the eastern part of the Alliance.”

The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

As of 2016, there were a reported 169,000 personnel in the Ukrainian Ground Forces. The Ukrainian Ground Forces are divided into Armoured and Mechanized Forces, Army Aviation, Army Air Defence and Rocket and Artillery Troops. There are 13 mechanized brigades and two mountain warfare brigades in the Mechanized Forces. Ukraine also has two armoured brigades. There are seven rocket and artillery brigades as well as five airmobile brigades. Until 2013, the Ground Forces were divided into three army corps. These were disbanded in 2013 and reorganized as Operation Command West, Operation Command North and Operation Command South. Operation Command East was formed in 2015 to coordinate forces in the war in Donbas.

In 2016, the Ukrainian Air Force was reported to have included 36,300 personnel.[69][70]

According to an August 2015 Kyiv Post report, the Ukrainian Navy consisted of 6,500 personnel.