US Defense Secretary Chuck Hagel said permanently stationing an additional U.S. Army brigade in Europe is among options to beef up security in the region. 

This is just one of the measures, as Russian troops remain massed along Ukraine’s eastern border.

The North Atlantic Treaty Organization has asked its top military commander, U.S. Air Force General Philip Breedlove, to study “a number of new possibilities, new measures, new options that we and NATO should consider,” Hagel said in an interview in Honolulu with Bloomberg News and Bloomberg Television.

Asked if those included permanently stationing a third brigade of 5,000 troops in Europe, Hagel said, “That’s all part of the measures that could be considered.”

Reports of Russia withdrawing some of its forces from the Ukrainian border are unconfirmed, he also said.

“There are troops that are moving around, we know that, but no indications of any significant movements of those large troop deployments along the border, away from the border,” he said.

As part of a U.S. defense strategy approved in 2012 by President Barack Obama, the Pentagon withdrew from Europe two of its four Army brigades, including 10,000 troops and their equipment, assuming Europe would remain peaceful and Russia would be a partner rather than an antagonist. 

Meanwhile, the Estonian ambassador to NATO said that the military airport in Amari, Estonia, will be converted into a base from which NATO will increase its surveillance of the Baltic air space. 

The move is a part of efforts on the part of NATO to ramp up the Baltic air policing mission it launched in 2004, Lauri Lepik told daily newspaper Postimees.

"Estonia has been saying for a long time that the Baltic air policing mission could have two bases - one in Siauliai, Lithuania, and another in Amari, Estonia," he told the newspaper.

"Several allies have offered to provide more aircraft, which makes it possible to open a second base," he said, adding that the construction of the base could be completed this year.

As previously reported, the US is also stepping up security in the Black Sea by extending the mission of USS Truxtun and sending a second destroyer, USS Donald Cook, which is due to arrive from Spain in “the next week or so”.

Pentagon is also sending additional 175 marines to Romania in an effort to strengthen its force of more than 300 marines already deployed in the country.