* Russia says troops leaving on trains, planes
* NATO says force on border is still very large
* Pentagon-Russian force capable of offensive operations (Adds comments and details on forthcoming exercises)
By Thomas Grove and Adrian Croft
MOSCOW/BRUSSELS, May 22 Russia said on Thursday it was moving troops and military equipment from border regions near Ukraine, but NATO said a large "coercive force" remained in place.
A withdrawal of forces from the border regions could ease tensions before Ukraine's presidential election on Sunday, which the United States and EU hope will strengthen the embattled central government.
Moscow has previously failed to keep promises to move troops back from the frontier with eastern Ukraine, where pro-Russian separatists have declared independence.
Ukraine and its Western allies view them as a potential invasion force, given Moscow's statements that it has the right to intervene in its former Soviet neighbour in order to protect Russian-speakers.
A NATO general said on Thursday that Russia was moving troops, though he said the size of the movement was unclear and that forces near the border remained a potential threat.
"The force that remains on the border is very large and it's very capable and remains in a very coercive posture," U.S. Air Force General Philip Breedlove, NATO's Supreme Allied Commander Europe, told a news conference.
NATO had previously put the number of Russian troops on the border at 40,000 but Breedlove said it was too early to classify their current size.
Russia's defence ministry said on Thursday 15 transport planes and 20 trains carrying personnel and military equipment had been moved out of the Rostov, Belgorod and Bryansk provinces bordering Ukraine after completing military exercises there.
It did not say how many troops were being moved or how many were staying behind.
"The moving of units is continuing, having completed planned exercises testing military preparedness in field conditions, to loading stations," a defence ministry statement said.
"TOO EARLY TO TELL"
Kiev has accused Moscow of sowing disorder in its mostly Russian-speaking east where pro-Russian separatists have declared independence and asked to join Russia.
Moscow denies destabilising Ukraine and has stopped short of endorsing the regions' independence or annexing the territories like it did Ukraine's Crimean peninsula in March.
The Pentagon acknowledged some movement of Russian forces off the border, but said it was only small numbers so far. It declined to offer estimates.
"It's too early to tell exactly what it all means. And the Russians still have a force along the border there that, frankly, is right now capable of even offensive military operations, should they choose to do so," Pentagon spokesman Colonel Steve Warren said.
The U.S. Ambassador to the OSCE Daniel Baer said the United States would welcome the withdrawal of troops but said current movements were not enough to confirm movement towards a complete removal of the forces.
He also criticised Russia's plans to hold air force "exercises" near the border with Ukraine on the same day as the presidential election.
"These are not the actions of a helpful de-escalating government," said Baer speaking in a teleconference with journalists around Europe from Vienna.
Russia's Defence Ministry plans to hold what it calls a competition of air force pilots that will include more than 50 jet fighters hitting ground targets and a display of manoeuvres to evade anti-aircraft rockets.
The competition is planned to be held in the Voronezh and Lipetsk provinces, less than 200 km (124 miles) from the Ukrainian border. (Additional reporting by Phil Stewart in Washington,; Editing by Mark Trevelyan and Philippa Fletcher)