(Reuters) -Russia warned on Tuesday of "catastrophic consequences" for Europe if the Ukraine war escalates, as NATO leaders prepared to deliver a "positive message" to Kyiv at a summit on its future prospects for joining the military alliance.

Moscow has cited NATO's eastern expansion as a key factor in its decision to invade Ukraine nearly 17 months ago. On Monday the Kremlin said that if Ukraine joined the alliance, this would pose a direct threat to Russia's security to which it would react clearly and firmly.

In a series of statements by senior Russian diplomats ahead of the NATO summit in Vilnius, Konstantin Gavrilov, a Vienna-based senior Russian security negotiator, accused the United States of fuelling the conflict by pouring arms into Ukraine.

In an interview with Russia's RIA state news agency, Gavrilov said Europe would be the first to face "catastrophic consequences" if the war escalated further. He did not specify what those consequences would be.

"Let's look at the facts - the fate of Europe is of little interest to the United States," he said, accusing Washington of pursuing an agenda of seeking to weaken and undermine Russia.

Earlier, Russia's ambassador to the United States, Anatoly Antonov, made a similar criticism of what he called Washington's "anti-Russian" stance at the summit in Vilnius.

"Everything is being done to prepare local public opinion for the approval of any anti-Russian decisions that will be made in Vilnius in the coming days," Antonov said in a post on the embassy's Telegram channel.

PLANS

At the summit in the Lithuanian capital, NATO leaders are set to approve the alliance's first comprehensive plans since the end of the Cold War to defend against any attack by Moscow.

Russia's ambassador to Belgium, Alexander Tokovinin, in comments cited by RIA, said those plans would make NATO's confrontation with Moscow more tense and prolonged.

Diplomats said differences were narrowing among the allies over Ukraine's push for NATO membership, though it will not be invited to join the alliance while the war still rages.

Gavrilov told RIA: "Both the United States and NATO understand that time is not working for them. They are losing in Ukraine," Gavrilov said.

Kyiv's counteroffensive, which began last month, has been proceeding more slowly than hoped, but Ukraine's military said on Monday its forces had caught occupying Russian troops "in a trap" in the shattered eastern city of Bakhmut.

Reuters could not immediately verify the battlefield claims.

(Reporting by Reuters; Writing by Gareth Jones; Editing by Lincoln Feast and Alex Richardson)