By Adrian Croft

BRUSSELS (Reuters) - NATO has detected signs of heightened Russian involvement in fighting in parts of eastern Ukraine, the alliance's top military commander said on Thursday.

U.S. Air Force General Philip Breedlove, NATO's supreme allied commander Europe, said fighting in the pro-Russian separatist uprising was now more intense in some places than it was before September's Minsk ceasefire agreement.

Breedlove told a news conference after a meeting of NATO defense chiefs that military leaders would try to re-establish contact with their Russian counterparts, broken off amid tensions over Ukraine.

Russia denies deploying troops in eastern Ukraine, where more than 4,800 people have been killed since a pro-Russian rebellion broke out soon after Moscow annexed Crimea in March. It also rejects accusations it is supplying arms.

"The situation along the line of contact in Ukraine is not good. The fighting has intensified to essentially pre-agreement or pre-standdown levels and in some cases beyond," Breedlove said.

Following that accord, NATO said Russia had pulled back some of the troops it had inside Ukraine supporting pro-Russian separatists; but in recent days, Ukraine has said there has again been an increase of Russian forces in the country.

Breedlove said he could not confirm Ukrainian President Petro Poroshenko's estimate of 9,000 Russian troops in eastern Ukraine.

"What we do see is that the Russian-backed forces have renewed capability now to bring pressure on the Ukrainian forces and have in several places moved the line of contact to the west and this is concerning," he said.

"We are beginning to see the (heat) signatures of air defense systems and electronic warfare systems that have accompanied past Russian troop movements into Ukraine," he said.

High-level military contact between Russia and NATO used to take place regularly until Russia's intervention in Ukraine.

The last contact took place last May between Gen. Knud Bartels, head of NATO's military committee, and General Valery Gerasimov, chief of the general staff of Russia's armed forces.

"We are going to re-establish that (communication), we have talked among several of us senior military leaders how we will do that ... but yes, we are going to re-establish communication with Valery (Gerasimov)," Breedlove said.

(Reporting by Adrian Croft; editing by Robert-Jan Bartunek and Ralph Boulton)