The United States is increasingly focused on how to secure Syria's chemical weapons if President Bashar al-Assad falls from power, Defense Secretary Leon Panetta said Thursday.

Panetta said he would not consider sending ground troops into the war-torn country, even to secure chemical sites, but he left the door open to some US military presence if Assad's downfall is followed by a peaceful transition.

While the US government has issued stern warnings to Damascus against resorting to chemical weaponry in its war with rebel forces, Panetta said the greater risk might be a chaotic vacuum if Assad is toppled.

"I think the greater concern right now is what steps does the international community take to make sure that when Assad comes down, that there is a process and procedure to make sure we get our hands on securing those sites," he said.

"That, I think, is the greater challenge right now."

The US government was discussing the issue with Israel and other countries in the region, he said, but ruled out deploying American ground forces in any "hostile" setting.

"We're not talking about ground troops," Panetta insisted, adding that any future US military role in Syria would only come about if a new government asked for assistance.

"You always have to keep the possibility that, if there is a peaceful transition and international organizations get involved, that they might ask for assistance in that situation," he said.

"But in a hostile situation, we're not planning for that."

The US military's top officer, General Martin Dempsey, told the same news conference that if Assad chose to use his chemical stockpiles against opposition forces, it would be virtually impossible to stop him.

He said preventing the use of chemical arms "would be almost unachievable... because you would have to have such clarity of intelligence, you know, persistent surveillance, you would have to actually see it before it happened."

"And that's unlikely, to be sure."

He said that clearly worded warnings to Assad from President Barack Obama have served as a deterrent.

Even if the regime chooses not to employ the weapons, the Obama administration worries that Islamist militants allied with rebel forces might gain control of some chemical sites.

Syria's chemical weapons stockpile, which dates back to the 1970s, is the biggest in the Middle East, but its precise scope remains unclear, according to analysts.

The country has hundreds of tons of various chemical agents, including sarin and VX nerve agents, as well as older blistering agents such as mustard gas, dispersed in dozens of manufacturing and storage sites, experts say.

But it remains unclear if the chemical weapons are mounted and ready to be launched on Scud missiles, if the chemical agents are maintained effectively, and whether the regime is able to replenish its chemical stocks.

Damascus has said it might use its chemical weapons if attacked by outsiders, although not against its own people.

Panetta's comments came as prospects for international diplomacy to halt the violence in Syria appeared bleak.

The regime blasted the UN-Arab League envoy, Lakhdar Brahimi, a day before he was due to hold talks with US and Russian officials, accusing him of "flagrant bias."

The 21-month civil war has claimed more than 60,000 lives, according to the United Nations.