Army defectors stand atop a tank in Hama in this still image taken from video (Reuters)
In Libya, we're carrying out an operation based on a clear UN mandate. We have the support of countries in the region. These two conditions are not met in Syria, Anders Fogh Rasmussen told France's Midi Libre regional daily.
Rasmsussen was quoted in French as condemning "Syrian security forces' violent acts that target the population" the day after almost 140 people died in what militants called one of the bloodiest days since protests started mid-March.
European nations have been calling for an urgent meeting of the UN Security Council, which has not yet been able to agree even on a statement on Syrian President Bashar al-Assad's crackdown against opponents.
Russia and China have threatened to veto any formal resolution against Assad.
Rasmussen, who is on holiday in the south of France, stressed that in Libya NATO's mandate was "to protect the population" and that it was "out of the question to deploy troops on the ground" to battle Moamer Gaddafi's forces.
"This is a restriction," Rasmussen said, nevertheless citing progress since the operation began four months ago.
"We've disrupted Gaddafi's war machine. The opposition is progressing. The regime's resources are diminishing. Ministers and generals are defecting."
Gaddafi's regime is "increasingly isolated", Rasmussen said. "The question isn't if Gaddafi will leave power but when," he said.
"My message (to Gaddafi) is double: 'Leave power and your place is in court at The Hague," Rasmussen said, referring to the International Criminal Court that in June issued arrest warrants for Gaddafi and other regime officials.
They are accused of crimes against humanity, committed since the bloody uprising started in mid-February.