* NATO says 20,000 Russian troops still close to border
* Large quantities of weapons being transferred-NATO
* Says Russia aiming to prevent defeat of separatists (Adds quotes)
By Adrian Croft
MONS, Belgium, Aug 28 (Reuters) - Well over 1,000 Russian troops are operating inside Ukraine, marking a significant escalation of Moscow's military involvement in the country, NATO said on Thursday.
The alliance released satellite images it said showed Russian combat forces, armed with heavy weapons, engaged in military operations inside Ukrainian territory.
"Over the past two weeks we have noted a significant escalation in both the level and sophistication of Russia's military interference in Ukraine," Dutch Brigadier-General Nico Tak, head of NATO's crisis management centre, told reporters at NATO's military headquarters near Mons, Belgium.
"We assess well over 1,000 Russian troops are now operating inside Ukraine," he said, referring to Russia's actions as "incursions" rather than an invasion. "They are supporting separatists (and) fighting with them."
NATO estimates Russia also has around 20,000 troops close to the Ukrainian border, Tak said.
"We have also detected large quantities of advanced weapons, including air defence systems, artillery, tanks and armoured personnel carriers being transferred to separatist forces in eastern Ukraine," he said.
Ukraine accused Russia on Thursday of bringing troops into the southeast of the country in support of pro-Moscow separatist rebels.
NATO ambassadors will hold an emergency meeting with their Ukrainian counterpart in Brussels on Friday at Kiev's request to discuss the situation, a NATO official said. NATO ambassadors will also meet separately on Friday.
One satellite picture released by NATO, taken on Aug. 21, showed Russian military units moving in convoy with self-propelled artillery in the area of Krasnodon, Ukraine, inside territory controlled by Russian separatists.
A second image, taken on Aug. 23, showed Russian self-propelled artillery units set up in firing positions near Krasnodon, supported by logistical vehicles likely to have been carrying extra ammunition and supplies, NATO said.
"This is highly sophisticated equipment which requires a well-trained crew. It takes months to train crews like that. It's extremely unlikely these sorts of units are manned by separatists," Tak said.
Tak said the escalation of Russian military operations in recent weeks was directly linked to the success of Ukrainian government operations against pro-Russian separatists in eastern Ukraine, including "the loss of territory by the separatists, the loss of weapons and the loss of manpower."
Since Monday, Russia had carried out a new incursion near Novoazovsk in south-eastern Ukraine, he said.
"This effectively creates a second front for the Ukrainian forces. This is an extremely effective way to take pressure off the separatists," he said.
The Russian move put Ukrainian armed forces in a dire situation because from south-eastern Ukraine Russian forces could either move to link up with Donetsk area or move further west towards Crimea, which Russia annexed in March, he said.
"From a strategic perspective it is clear that Russia is not willing to accept a defeat of the separatists. So it will most likely do anything that it takes to prevent such a defeat," Tak said.
"I suspect they (Russian forces) will do no more than absolutely necessary at this point to show their hand," he said.
"The next thing we will see is Russia is going to try and freeze this conflict. It is going to try and prolong the conflict, making it difficult for Ukraine to sustain that operation, making sure that they don't have to sell a defeat at home and trying to broker some kind of deal with Kiev, something that will allow them enduring influence in eastern Ukraine," he said.
Asked about the likelihood of Russian troops creating a "land bridge" from Russia to Crimea via Mariupol, Tak said: "It is obviously something that we are watching quite closely."
Creating a land bridge to Crimea would require a lot more Russian resources but it was not impossible, he said.
Tak said there was clear evidence that there had been military contact between Ukrainian and Russian forces.
Pictures of graves in northwestern Russia that had appeared in the media suggested Russia had taken casualties, he said.
Despite NATO's concern over the situation in Ukraine, the 28-nation, U.S. dominated alliance has said repeatedly it has no intention of intervening militarily in Ukraine, which is not a NATO member.
It has reinforced the security of NATO allies in the region and will discuss further steps at a summit in Wales next week. (Editing by Ralph Boulton)