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Although the decision did not bolster US support to Ukraine, which is not part of the Nato alliance, it underscored the growing Nato preparations for what Washington and Kyiv believe are Russian moves to mass its forces for a potential invasion of Ukraine.
Pentagon spokesman John Kirby said the bulk of the 8,500 US troops were being notified of ready-to-deploy orders so they could fill in the ranks of a Nato rapid response force, should the alliance call them up for duty.
But Kirby stressed that Austin also wanted an unspecified number of troops "to be ready for any other contingencies as well."
"What's happening now is getting them ready on a shorter tether," Kirby told a news briefing.
"Today, we're not talking about deployment orders. We have no deployment orders to speak to."
The alliance describes the Nato Response Force (NRF) as "a highly ready and technologically advanced multinational force made up of land, air, maritime and Special Operations Forces (SOF) components that the alliance can deploy quickly, wherever needed."
Nato enhanced the NRF in 2014 by creating a "spearhead force" within it, known as the Very High Readiness Joint Task Force.
The US troops notified of ready-to-deploy orders on Monday included additional brigade combat teams, logistics personnel, medical support, aviation support, and troops forces involved with intelligence, surveillance and reconnaissance missions.
The announcement came the same day Nato said it was putting forces on standby and reinforcing eastern Europe with more ships and fighter jets, and could also send additional troops to its south-east flank, in what Russia denounced as an escalation of tensions over Ukraine.
So far, Nato has about 4,000 troops in multinational battalions in Estonia, Lithuania, Latvia and Poland, backed by tanks, air defenses and intelligence and surveillance units.
Russia denies planning an invasion. But, having engineered the crisis by surrounding Ukraine with forces from the north, east and south, Moscow is now citing the Western response as evidence to support its narrative that Russia is the target, not the instigator, of aggression.
Kirby said Russia could easily de-escalate the crisis by pulling back forces. But that didn't appear in the cards, he said.
"It's very clear that the Russians have no intention right now of de-escalating," Kirby said.