The United States has heightened the readiness of some 8,500 U.S. troops amid tensions between Russia and NATO over a potential invasion of Ukraine, but no decisions have been made yet to deploy them.

"Secretary Austin has placed a range of units in the United States on a heightened preparedness to deploy, which increases our readiness to provide forces if NATO should activate" the NATO National Response Force, said Pentagon press secretary John Kirby on Monday.

The NATO NRF is a multinational rapid response force comprised of land, air, maritime and Special Operations Forces.

Russian President Vladimir Putin gestures while speaking at an extended meeting of the Russian Defense Ministry Board at the National Defense Control Center in Moscow, Russia, Dec. 21, 2021. (Sergey Guneyev, Sputnik, Kremlin Pool Photo via AP, File)

"The United States is deeply concerned about the current situation in Europe," Kirby said. "We remain keenly focused on Russia's unusual military activities near the Ukrainian border, including in Belarus, and consulting extensively with our transatlantic allies and partners. The department continues to support diplomatic efforts to deescalate the situation.

"Now, as the president has said, even as we continue to prioritize diplomacy and dialogue, we must also increase readiness in support of its obligations to the security and defense of NATO and the security of its citizens abroad," he continued.

POMPEO: RUSSIA-UKRAINE CONFLICT PUTS BIDEN ADMINISTRATION'S WEAKNESS ON FULL DISPLAY

President Joe Biden speaks during a news conference at the NATO summit at NATO headquarters in Brussels, Monday, June 14, 2021. (AP Photo/Patrick Semansky)

The Pentagon's announcement comes the same day that NATO said that it will send fighter jets and ships to Eastern Europe.

NATO SENDING FIGHTERS, SHIPS TO REGION; UK TO WITHDRAW SOME STAFF FROM EMBASSY IN UKRAINE

Denmark is sending a frigate to the Baltic Sea and four F-16 fighter jets to Lithuania. Spain is also sending ships to join NATO naval forces in the region and is considering sending fighter jets to Bulgaria, the statement read. France said it is open to sending troops to Romania, and the Netherlands also said it will send two F-35 fighter aircraft to Bulgaria in April.

Danish Air Force F-16 takes off in Monte Real Air Force Base during Real Thaw 2018 exercise on February 06, 2018 in Monte Real, Portugal. (Photo by Horacio Villalobos - Corbis/Corbis via Getty Images)

BLINKEN DODGES QUESTION ON SENDING U.S. TROOPS TO UKRAINE

Last week, U.S. officials told Fox News that the State Department is ordering families of U.S. Embassy staff in Kyiv, Ukraine, to begin evacuating, and that they will encourage Americans to leave the country on commercial flights while they remain available.

Moscow has massed tens of thousands of troops along the Ukraine border, prompting fears of invasion.

Fox News' Edmund DeMarche, Lucas Tomlinson and The Associated Press contributed to this report.