The United States has accelerated the deployment of troops to Poland, the Baltic states and Romania as part of raising security in the region, officials said.
Polish Defence Minister Antoni Macierewicz made the announcement on Wednesday following talks with the commander of US land troops in Europe, Lieutenant-General Ben Hodges.
American troops will be deployed in early January - instead of late in the month - to Poland, the Baltic states, and Romania, Hodges said.
"I am very happy that a decision has been taken by the US side for an earlier deployment," Macierewicz said.
An armoured brigade combat team from Fort Carson, Colorado will be based in Zagan, western Poland, while another US force, a battalion, will be deployed on April 1 to Orzysz, in the northeast.
"I'm confident in the very powerful signal, the message it will send [that] the United States, along with the rest of NATO, is committed to deterrence," Hodges said.
"I'm excited about what my country is doing and I'm excited about continuing to work with our ally, Poland."
Poland and the Baltic nations of Lithuania, Latvia, and Estonia have been uneasy about increased Russian military operations in the region, especially after its 2014 annexation of Crimea from Ukraine, and have requested US and NATO troops' presence on their territory as a deterrent.
The alliance and the US insist the troop presence is not aimed against anyone, but Russia has threatened measures in response.
"We must clearly grasp what is happening around our borders, why NATO is building up arms and infrastructure. It deploys four additional battalions to alliance member states in Eastern Europe. Why are they doing it?" asked Nikolay Bordyuzha, secretary-general of the Collective Security Treaty Organisation (CSTO) - a Russian-led military bloc that includes Armenia, Belarus, Kazakhstan, Kyrgyzstan and Tajikistan.
"We cannot remain silent and do nothing, looking at how countries around us are being crammed with weapons, and troops being moved," he told the Russian newspaper Izvestia on Tuesday.
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He also said the leaders of the CSTO member states would gather on December 26 in St Petersburg, to discuss possible responses to NATO's latest moves.
In November, Russia said it was installing S-400 surface-to-air missiles and nuclear-capable Iskander systems in the exclave of Kaliningrad that borders Poland and Lithuania in retaliation for the NATO deployment of its so-called defence shield in Eastern Europe.
The Kremlin says the shield's aim is to neutralise Moscow's nuclear arsenal long enough for the United States to strike Russia in the event of war. Washington and NATO deny that.
Source: News agencies