NATO chief Anders Fogh Rasmussen on Sunday voiced concern about violence in Syria, but said the alliance has "no intention" of taking military action against the regime.
"We strongly condemn the behavior of the Syrian security forces and their crackdowns on the Syrian population and we urge the Syrian leadership to accommodate the legitimate aspirations of the Syrian people," he told a news conference during an alliance summit in Chicago.
"But again NATO has no intention to intervene in Syria."
NATO governments have come under criticism for backing an air war in Libya but ruling out military intervention in Syria, where opposition demonstrators and badly outgunned rebels have been hammered by heavily-armed regime forces.
Signup to our newsletter and follow us on Facebook and Twitter!
Rasmussen spoke a day after G8 nations said a "political transition" was needed to end the crisis in Syria, where monitors say more than 12,000 people have died in a government crackdown since March 2011.
The NATO secretary-general urged the Syrian regime to comply with a UN ceasefire and peace plan, saying it was "the best platform for finding a solution in Syria."
The United States is supplying communication equipment and night-vision goggles to Syrian rebels, but so far has stopped short of openly arming the rebels.
US media have reported Washington has started to coordinate deliveries of weapons to rebels financed by Gulf states.
In Syria, a rocket-propelled grenade exploded near a team of UN observers in a Damascus suburb Sunday, the military said, as dozens of people were killed in violence while clashes raged between regime troops and armed rebels.