The top US military commander in Europe said on Tuesday that several Nato countries are working on plans for military action inside Syria.
Admiral James Stavridis told the US Senate armed services committee that a number of Nato nations are looking at a variety of military operations to end the deadlock and assist the opposition forces.
Such operations could include using aircraft to impose a no-fly zone, providing military assistance to the rebels and imposing arms embargoes.
As with US and international involvement in Libya in 2011, a resolution from the UN security council and agreement among the cold-war alliance's 28 members would be necessary before Nato assumes a military role in Syria, he said.
"We are prepared if called upon to be engaged as we were in Libya."
Within individual member countries, the admiral added, "there's a great deal of discussion" about lethal support to Syria, no-fly zones, arms embargoes and other forms of action.
"It is moving individually within the nations, but it has not yet come into Nato as an overall Nato-type approach," he said.
Senate armed services chairman Carl Levin asked whether there was any consideration of targeting Syria's air defences.
Admiral Stavridis simply said: "Yes."
Nato has installed Patriot missile batteries in southern Turkey along the border with Syria that are capable of shooting down aircraft.
During an exchange with Senator John McCain, Adm Stavridis said that the Patriots could be positioned in such a way as to shoot down Syrian aircraft and he indicated that doing so would be a powerful disincentive for pilots to fly in that area.
He said that his personal opinion was that providing military assistance to the Syrian opposition "would be helpful in breaking the deadlock and bringing down the Assad regime."
Following the hearing, Republican Senator Lindsey Graham talked of deploying US troops to Syria to secure the country's chemical weapons stockpile.
"If the choice is to send in troops versus allowing chemical weapons to get into the hands of some of the most violent people in the world, I vote to cut this off before it's a problem," he told reporters.
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