The United States is not asking NATO to back a no-fly zone in Syria and the issue is not currently on the alliance's agenda, the outgoing US ambassador to NATO said on Monday.

The purpose of such a move would be to prevent Syrian President Bashar al-Assad from using air power against rebel forces, thereby addressing a critical imbalance in the civil war. But it would be costly to enforce, and could mean entering the conflict by destroying Syria's Russian-built air defences.

"We are not pushing for a no-fly zone at NATO," Ambassador Ivo Daalder said in response to a question after his farewell speech in Brussels.

As of today, he said: "the issue of a no-fly zone is not on the table at NATO. Whether it will (be) tomorrow or some other day, I don't know, but it isn't there yet. It isn't, as far as I know, on the table of any NATO member, including, so far the United States."

US President Barack Obama's administration said last week it would arm Syrian rebels, having obtained proof the Syrian government used chemical weapons against fighters trying to overthrow Assad.

Western diplomats said last Friday Washington was considering a limited no-fly zone over parts of Syria. But the White House noted later that it would be far harder and costlier to set one up there than it was during the conflict in Libya which led to the overthrow of Muammar Gaddafi in 2011.

It said the United States had no national interest in pursuing that option.

Daalder said the United Nations would have the main responsibility for dealing with the humanitarian consequences if the Assad government collapsed, but he said NATO might agree to help by bringing in equipment by air if the United Nations asked it to do so.