The United States on Monday said it had placed 8,500 troops on "heightened alert" over the Ukraine standoff, saying that Russia had continued to build its force presence on the country's border.

Pentagon spokesman John Kirby said the troops could be ordered to deploy in support of the quick-reaction NATO Response Force if it is activated to bolster Eastern European members of the alliance due to Russia's threat.

"It's very clear that the Russians have no intention right now of de-escalating," Kirby told reporters as he announced the troop alert.

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The troops, mostly ground forces and support units, are being told to be ready to move out within five days if an order to deploy is given, he said.

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"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," he said.

Kirby said that if the NRF is mobilized, the US troops could be deployed to buttress the strength of national forces in NATO allies on Russia's border.

"The bulk of them are intended for the NATO Response Force," he said.

US Secretary of Defense Lloyd Austin could also deploy some for "other contingencies," he said, without giving specifics.

But he said they were not to be deployed to Ukraine, which seeks to be but is not a member of the alliance.

"The actions that Russia appears to be taking to threaten its neighbor further and to potentially violate further Ukraine's territorial integrity certainly make things more tense on the European continent," Kirby said.

"We absolutely remain committed to bolstering the capabilities of NATO's eastern flank, to the degree that they desire that extra support."

He said that Russia continues to ramp up its force presence on Ukraine's border, including in neighboring Russian ally Belarus.

The Russians are estimated to have placed more than 100,000 troops and heavy military equipment within striking distance of Ukraine.

"We're watching it very, very closely," Kirby said.

"We believe that we have a pretty good sight picture on what they have there, and what they continue to add, both in terms of the western part of Russia as well as Belarus," he said.

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