NATO Secretary-General Jens Stoltenberg is calling on Russia to play a constructive role in restoring peace in war-ravaged Syria.

Stoltenberg, in New York to attend the annual U.N. General Assembly, told The Associated Press on Tuesday that the alliance welcomes Russia's plans to combat the Islamic State group in Syria.

Support for President Bashar Assad's regime, however, is not "a constructive contribution to finding a political solution," he stressed.

In his address Monday at the U.N., Russian President Vladimir Putin urged the world to back Assad, saying the failure to engage the Syrian military in the fight against Islamic State militants was a "huge mistake."

On Ukraine, where Russian has supported a rebellion in the east of the country and annexed Crimea, Stoltenberg said Putin's speech provided no new signals. He noted positive signs on the ground, however, given the cease-fire had mainly held.

But Moscow's continued backing for the separatists was, he said, the "main challenge" to ending the conflict, as are rebel plans hold local elections in defiance of the Ukrainian government.

While NATO, the European Union and the United States stand behind Ukraine in the conflict, they have rejected calls for offensive weapons. In the most recent development, Russia, Ukraine and the Organization for Security and Cooperation in Europe reached a deal late Tuesday on the withdrawal of tanks and other weapons from the frontline.