NATO military action in Syria is not on the table despite the massacre of civilians, allegedly by pro-government foces, the US envoy to the alliance said on Thursday.

NATO allies have neither discussed an intervention in Syria nor made any military planning to stop the relentless crackdown by the regime of President Bashar al-Assad against dissidents, said US ambassador Ivo Daalder.

Daalder noted that the alliance launched its air war in Libya last year after three conditions were fulfilled: a "demonstrable need" to intervene, support from nations in the region, and a UN Security Council mandate.

"With respect to a demonstrable need, clearly when government forces are attacking civilians with artillery and tanks, there is a need to bring that to an end.

That was true in Libya and that is true in Syria," he said.

But there is neither regional support nor a UN mandate to act militarily in Syria.

"So under those circumstances, the NATO countries understand that the issue of military intervention, which is also always complex, is not right now on the table when it comes to Syria," Daalder said.

All NATO members, notably Syria's neighbour Turkey, are watching the situation "very carefully and that is where things stand right now," the ambassador added.

"How it will evolve in the future is anyone's guess," he said.

"But the point is that for now there is no active planning in NATO for a military intervention and there is no agreement among or even within the NATO members for moving in this direction at this point."

French President Francois Hollande said Tuesday that the use of armed force could be possible in Syria following a massacre in the town of Houla last week, but that it had to be carried out under UN auspices.

World outrage over the situation in Syria intensified after at least 108 people, many of them children, were killed at Houla. [AFP]