By Louis Charbonneau

UNITED NATIONS (Reuters) - Lithuania's foreign minister confirmed on Thursday reports that Russia has been massing troops and weaponry near the border of Ukraine and urged pressure on Moscow until it stops supporting Ukrainian rebels.

Reuters reported this week that Russia's army was massing troops and hundreds of pieces of weaponry including mobile rocket launchers, tanks and artillery at a makeshift base near the border with Ukraine.

Asked about that report, Lithuanian Foreign Minister Linas Linkevicius said it was true that Moscow was moving troops and hardware in the vicinity of Donetsk, Mariupol and elsewhere.

"We are concerned about these concentrations of troops, and these are big numbers," he said in an interview. "These groups, the separatists, are equipped better than some European armies."

Lithuania, a former Soviet republic, is a member of NATO and the European Union. It is on the United Nations Security Council through the end of this year.

Earlier in May, NATO military commander General Philip Breedlove said he believed the separatists were taking advantage of a ceasefire that went into effect in February to re-arm and prepare for a new offensive, but he gave no details.

Linkevicius echoed Breedlove's remarks and said the Russian-backed separatists in Ukraine had used the lull in fighting to regroup and reposition, not for de-escalation.

Russia has denied that its military is involved in the conflict in Ukraine's east, where separatists have been fighting forces loyal to the pro-Western government in Kiev.

Russia's defense ministry said it had no immediate comment when asked about the build-up of troops and weapons.

Linkevicius said it was vital to keep pressure on Russia in the form of sanctions and diplomatic isolation.

"If we will not continue our pressure ... that would be a signal (for Russia) to act," he said. "If they will not stop supporting, stop delivering weapons, including heavy weapons, tanks ... we cannot talk about any progress, any change in policy."

(Reporting by Louis Charbonneau; Editing by Toni Reinhold)