NATO has vowed to hold "continuous" military exercises in eastern Europe to deter Russia but has yet to respond to a request by Baltic states for a permanent brigade, Lithuania said Wednesday.

In May, Lithuania, Latvia and Estonia formally asked NATO's top commander to deploy a "permanent rotational" battalion-sized unit in each country amid concerns triggered by Russia's military resurgence.

NATO has so far refused to approve a substantial permanent deployment, with some saying it could breach a 1997 agreement with Russia and trigger an arms race.

Lithuania's military spokesman Captain Mindaugas Neimontas confirmed to AFP Wednesday that US General Philip M. Breedlove sent Vilnius a classified document "several weeks ago" vowing that NATO's military drills would continue without considerable breaks.

"NATO's military exercises programme in the region (will be) continuous, that is, exercises will be held without major intervals," he added.

Lithuania's Foreign Minister Linas Linkevicius told AFP Wednesday that "even if allied forces are not called permanent but are permanently rotated, we are happy about it."

"Discussions about numbers and the size of units are continuing at the working level," he said.

Lithuania's top military man, Lieutenant General Jonas Vytautas Zukas, said he was happy that "NATO troops will be training in the Baltic Sea and on its land without major interruptions".

"We want the required level of deterrence in our region to be ensured on a continuous basis. How this goal is achieved is not that important", he said in a Wednesday statement.

Currently, Lithuania is hosting over 600 troops from the United States, Germany, Portugal, Norway and Italy participating in military drills and NATO's long-term Baltic air police mission.

Last month, the US pledged to deploy unmanned heavy weapons, including tanks, in the Baltic states as well as Bulgaria, Romania and Poland.

Russia has denounced the NATO moves as Cold War-style provocations while upgrading its own armed forces, including a pledge to deploy more than 40 new nuclear ballistic missiles this year.

The Baltic republics with a combined population of six million were annexed by the Soviet Union in 1940 and remained under Moscow's thumb until 1991.

They joined the EU and NATO in 2004 and now fear that Moscow could try to destabilise them to test NATO's commitment to collective defence.

Russia's Prosecutor General's office said Tuesday it was reviewing the legality of the independence of the three Baltic countries, drawing furious reactions from Baltic leaders.

The Kremlin and Russian foreign minister said they were unaware of the initiative.