KYIV, Aug. 25 - U.S. National Security Adviser John Bolton said on Friday during a trip to Kiev that Ukraine had made progress in its efforts to join the North Atlantic Treaty Organization (NATO), but that it still had more work to do, Reuters reported.

Russia strongly opposes NATO expansion towards its western borders and in 2014 annexed the peninsula of Crimea from Ukraine following a pro-Western revolution there.

Bolton said it was important to resolve the Ukraine crisis and that it would be dangerous to leave the situation as it is in Crimea and eastern Ukraine where Moscow has backed separatists in a conflict against Kyiv.

Russian Prime Minister Dmitry Medvedev said earlier this month that any future NATO decision to admit Georgia or Ukraine to its ranks could trigger “a terrible conflict” and questioned why the alliance would consider such a move.

“This could provoke a terrible conflict. I don’t understand what they are doing this for,” Medvedev told Russia’s Kommersant newspaper in an interview.

Georgia’s and Ukraine’s NATO ambitions have been a source of anger for Russia - which shares a border with ex-Soviet republics and does not want to see them join what it regards as a hostile military bloc - since 2008 when NATO leaders promised Georgia and Ukraine they would one day join the alliance.

Russian troops occupied Ukraine’s Crimean peninsula in February 2014, eventually leading to Moscow’s move to annex the peninsula in March 2014. Russian troops, along with local separatists, also occupied and control certain parts of Donetsk and Luhansk regions in eastern Ukraine since June 2014.

NATO leaders discussed ties with Georgia at their summit in Brussels in July, a move Medvedev, who was president when Russian and Georgian forces clashed in 2008, condemned.

“As for the recent NATO decision reaffirming its commitment to eventually admit Georgia, what can I say to this? It is an absolutely irresponsible position and a threat to peace,” said Medvedev. (rt/ez)