The tiny Balkan state had made "truly remarkable" progress with reforms aimed to meet alliance standards, U.S. Permanent Representative to NATO Victoria Nuland said.
"But our message today is, when you are running the marathon you cannot relax" just before the end, she said. "You have to sprint to the finish."
Macedonia and fellow-Balkan states Albania and Croatia hope to be invited to join NATO at the alliance's summit next spring in Bucharest, Romania.
After a meeting with Macedonian Prime Minister Nikola Gruevski, Nuland urged further progress with reforms aimed at reconciliation with the country's restive ethnic Albanian minority, members of which staged an armed uprising in 2001.
Nuland added that Macedonia has to strengthen the rule of law, reform the judiciary and promote good relations with its neighbors — particularly NATO-member Greece. Athens has threatened to block Macedonia's NATO bid due to a dispute over the country's name, which Greece says could imply claims on the northern Greek province of Macedonia.