US troops will remain in Poland and the Baltic states through 2015 "to deter Russian aggression", the top US commander for Europe said on Sunday.

"There are going to be US army forces here in Lithuania as well as Estonia and Latvia and Poland for as long as it's required to deter Russian aggression," said Lt. General Ben Hodges, commander of US Army Europe.

"We have planned rotations out through next year. Units are designated that will continue to do this," he told reporters in the Lithuanian capital Vilnius.

The United States sent about 600 troops in April to the ex-Soviet Baltic states and former Warsaw Pact member Poland in response to tension with Russia over the war in Ukraine.

European NATO allies have also deployed hundreds of troops for exercises in recent months. The alliance has also boosted its airforce presence in the region.

Hodges said NATO should guard its unity at a time when Russia is "trying to intimidate" it's neighbours with military drills and frequent violation of their airspace.

"The discipline, unity of the nations is really very important at this point, so that a small incident doesn't lead to something that didn't have to happen," the general said.

NATO has accused Moscow of sending troops to fight alongside separatists in eastern Ukraine. Moscow denies involvement in the fighting, although it openly gives the rebels political backing and non-lethal supplies.