NATO chief Jens Stoltenberg urged Macedonia on Thursday to keep up newfound momentum towards solving a 27-year-old dispute with Greece over its name, a key condition for joining the alliance.

"I welcome the willingness of your government to resolve this issue and the resolve the government has shown," Stoltenberg told Macedonia's parliament in Skopje during a two-day visit, the first by a NATO chief since 2014.

A two-hour meeting on Wednesday of negotiators on the issue at the United Nations in New York was a "valuable step", Stoltenberg said.

"I am encouraged by recent progress – as are all NATO Allies – and I urge you to take this chance to move forward."

Greece blocked Macedonia from joining NATO 10 years ago because of the row dating from the 1991-92 breakup of Yugoslavia.

Athens argues that the name Macedonia suggests that Skopje has territorial claims to the northern Greek region of the same name.

After the talks in New York, UN envoy Matthew Nimetz said he was "very hopeful" that a solution was within reach.

The two sides resumed talks on the issue after a change of power in Skopje last year, when a social democratic party replaced conservatives led by nationalist Nikola Gruevski following snap elections.

"There is no other way to join NATO without solving the name issue," Stoltenberg said after meeting Prime Minister Zoran Zaev later Thursday.

But "NATO doors remain open," he added.

Zaev said earlier this month that he believed a solution could be found by July.

Stoltenberg also commended Macedonia for reform efforts including "important progress on transparency, accountability, oversight of the intelligence and security agencies, and judicial reform".