SKOPJE, Macedonia — A senior United States official on Tuesday urged Macedonia and Greece to seize the moment to resolve a decades-long dispute over the Balkan country's name that has prevented it from joining NATO.

Assistant Secretary of State for European and Eurasian Affairs Wes Mitchell visited Macedonia's capital, Skopje, as part of a Balkan tour. He told media "there is a tremendous opportunity" for Skopje and Athens to reach a deal over the long-running spat.

Macedonia and Greece have been at odds for a quarter-century over the name Macedonia, but have pledged to resolve the dispute by the summer. Greece argues the country's name harbors territorial claims on its own northern province of Macedonia.

The two countries have intensified talks recently in hopes of finding a compromise to end the dispute.

Prime Minister Zoran Zaev, speaking at a joint news conference with Mitchell, said he had told the U.S. official Macedonia and Greece were "starting the final phase that will ultimately bring a dignified solution to this issue."

The left-led governments in both countries have been holding talks with a United Nations negotiator, Matthew Nimetz.

Greece wants Macedonia to add a modifier such as "north" or "upper" to Macedonia's name, and Zaev has said he is open to some options.

But the proposals have triggered protests in both countries.

"We acknowledged that the only solution that will last is the one that both parties would work out for themselves," Mitchell said. "We are encouraged in that regard with the steps that Macedonia and Greece have taken, so we want to be supportive."

Mitchell visited Kosovo before arriving in Macedonia, and was scheduled to head from Skopje to Serbia.