Matthew Nimetz said the meeting had left a "very positive feeling" and that progress had been made on "some of the less controversial aspects of an agreement".
Nevertheless, while "difficult issues were addressed, there still are difficult issues to resolve," he warned.
"A series of meetings will go on in the future," he said.
The spat, which has sparked emotional protests by thousands of people in recent weeks in both countries, has raged since Macedonia became an independent country in 1991.
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Greece puts forward five alternative names for Macedonia
Greece objects to its northern neighbour's name, arguing it suggests that Macedoniahas claims to the territory and heritage of Greece's historic northern region of the same name.
In light of Greece's objections, the country joined the United Nations in 1993 with the unwieldy name of the Former Yugoslav Republic of Macedonia, or FYROM for short.
The dispute has also hampered Macedonia's hopes of joining the European Union and the NATO military alliance.
Last year Nimetz, a 78-year-old veteran US diplomat long involved in the row -- reportedly for a token $1 a year -- was able to relaunch the process, meeting both sides separately and together.
'Honourable compromise'
There have been signs of progress, with Greek Foreign Minister Nikos Kotzias and his Macedonian counterpart Nikola Dimitrov saying last week they were looking for an "honourable compromise".
Several possible new names have been doing the rounds, with "Gorna Makedonija" ("Upper Macedonia") the most frequently mentioned, although Greece insists it be "untranslatable" and not referred to as "Macedonia" in other languages.
Macedonia made a concession in February by changing the name of the capital's Alexander the Great airport to Skopje International Airport.
The motorway linking Macedonia with Greece has lso just been renamed the Friendship Highway.
Athens and Skopje have a "common resolve... to solve the problem," Macedonian political scientist Nano Ruzin, a former NATO ambassador close to the government, told AFP.
The nationalist problem
But both countries' governments are facing the opposition of nationalists who refuse any concession.
Kotzias has received threats and a survey Sunday showed that seven out of 10 Greeks would be hostile to a name including the term Macedonia or a derivative.
In Skopje, the nationalist right-wing opposition VMRO-DPMNE party could use the issue to weaken the government, which relies on a thin majority.
And the devil is in the detail.
Athens wants the change of name to be backed with a constitutional change and for it be applied on the "erga omnes" ("towards all") legal principle, meaning universal use inside and outside Macedonia.
But even with the support of the ethnic Albanian minority parties, the Social Democrats who lead Macedonia's government do not have a majority to pass a constitutional change in parliament.
Greek public television ERT quoted Kotzias as saying on Friday that the "erga omnes" issue and the constitutional changes remained the "toughest part" of the negotiations.
Kotzias and Dimitrov will meet again soon after April 8 and if that goes well, the two countries' prime ministers could follow suit, ERT said.
