US troops attend welcoming ceremony for NATO troops near Orzysz, Poland. (Reuters file photo)

The United States is further ratcheting up tensions with Russia, announces to put about American 8,500 troops on heightened alert for a possible deployment to Eastern Europe, amid claims that Moscow is preparing to invade Ukraine.

The United States will act firmly in defense of its national interests in response to actions by Russia that harm us, our allies, or partners, Pentagon spokesman John Kirby told reporters on Monday.

US Defense Secretary Lloyd 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 Response Force or if other situations developed, at the direction of President Joe Biden.

All told, the number of forces that the secretary has placed on heightened alert comes up to about 8,500 personnel, Kirby said.

Kirby stressed that no final decision to deploy the troops had been made and the actions are about getting troops ready.

We�re going to be ready, we�re going to be prepared to help bolster our allies with capabilities they might need, Kirby said. We�re going to do this in lockstep with them ... this is really about reassuring the Eastern flank of NATO.

The United States has also reportedly allowed three fellow-NATO member states in the Baltic region to send US-made missiles and other armaments to Ukraine.

The US State Department has approved shipments of US-made missiles and other weapons from NATO allies Lithuania, Latvia and Estonia to Ukraine, Reuters cited three unnamed sources as saying on Wednesday.

The United States sent two weapons shipments to Ukraine over the past week amid Washington�s claims that Russia is preparing to invade the neighboring country. Moscow has rejected the accusations.

NATO countries are also sending additional ships and fighter jets to Eastern Europe and putting forces on standby.

In a statement on Monday, NATO Secretary-General Jens Stoltenberg welcomed the move, adding that the Western military alliance will continue to take all necessary measures to protect and defend all allies, including by reinforcing the eastern part of the alliance.

We will always respond to any deterioration of our security environment, including through strengthening our collective defense, he stated.

The New York Times also reported on Monday that the Biden was considering sending thousands of troops, as well as ships and fighter jets, to NATO allies in the region.

A number of NATO allies have made announcements regarding current or upcoming deployments in recent days, pointing to heightened tensions with Russia.

Moscow was quick enough to hit back at NATO�s announcement, warning the military alliance that Russia will respond appropriately if the US boosts troops in Eastern Europe and the Baltic countries.

Russian lawmaker Andrei Kartapolov, who heads the Russian parliament�s defense committee, raised the alarm over the alliance�s expansion in Eastern Europe and urged the US not to fuel tensions.