LONDON - British Prime Minister David Cameron said he'd support giving President Bashar Assad safe passage out of Syria if it ended the 20-month civil conflict.

"Anything, anything to get that man out of the country and to have a safe transition in Syria," Cameron told Al Arabiya television in Abu Dhabi on Tuesday. "I am certainly not offering him an exit plan to Britain, but if he wants to leave, he could leave, that could be arranged."

He said he would prefer to see Assad face "the full force of international law and justice for what he's done." Britain is looking at what else it could do to support Syrian rebels short of arming them, which he's not currently planning to do, he said.

'I am very frustrated'

More than 35,000 people have died since the uprising began, according to the opposition Syrian Observatory for Human Rights. Cameron has urged more international action to support the rebels and topple Assad, blaming China and Russia for blocking such moves at the United Nations.

"I am very frustrated that we can't do more," Cameron said. "This is an appalling slaughter that is taking place in our world today -- 40,000 lives lost already and you can see, on your television screens, night after night, helicopters, airplanes belonging to the Assad regime pounding his own country and murdering his own people."

He warned that a failure to end the conflict could store up trouble for the future.

"My fear is, firstly, that the slaughter will continue, that the loss of life will continue," he said. "That should be our number one concern. But there is another fear, which is that the longer this goes on, the more that it can promote and drive extremism and we'll see instability in the region as well."

In an interview with Sky News Arabia, he acknowledged that there would be "steps backward" among countries that had overthrown their governments, including Libya.

"I worried about extremists before the Arab Spring," he said. "I worried about them during the Arab Spring. I'm worried about them now. There's no straight line between what those countries had and a peaceful democratic future. These things are difficult, they take time. There will be steps forward then steps backward."

'What more can we do?'

Cameron will be discussing the situation in Syria on his tour of the Middle East, which on Tuesday saw him travel from the United Arab Emirates to Saudi Arabia.

"We must ask ourselves what more can we do: how can we help the opposition?" he said. "How can we put the pressure on Assad? How can we work with partners in the region to turn this around?"

Lakhdar Brahimi, the U.N. peace envoy for Syria, made similar remarks in an interview published in the pan-Arab Al Hayat newspaper. If the crisis is left unresolved, Syria may descend into warlordism, with "the total collapse of the state and Syria turning into a new Somalia," he said. Somalia has been without effective central government since civil war erupted in 1991.