The top US military commander in Europe has said that several Nato countries are working on contingency plans for possible military action to end the two-year civil war in Syria.

The claim came as President Bashar Assad's regime accused US-backed Syrian rebels of using chemical weapons.

The Obama administration rejected the Assad claim as a sign of desperation by a besieged government intent on drawing attention from its war atrocities - some 70,000 dead, more than a million refugees and 2.5 million people internally displaced.

A US official said there was no evidence that either Assad forces or the opposition had used chemical weapons in an attack in northern Syria.

As the war enters its third year, the US military, State Department officials and the UN high commissioner for refugees delivered a dire assessment of a deteriorating situation in Syria and the sober view that even if Assad leaves, the Middle East nation could slip into civil strife similar to the Balkans in the 1990s.

"The Syrian situation continues to become worse and worse and worse," Admiral James Stavridis, the commander of US European Command, told the Senate Armed Services Committee. "No end is in sight to a vicious civil war."

He said a number of Nato nations are looking at a variety of military operations to end the deadlock and assist the opposition forces, including 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 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," he said.

His personal opinion is that providing military assistance to the Syrian opposition "would be helpful in breaking the deadlock and bringing down the Assad regime".

The Press Association, photo by NATO IMS