WASHINGTON (AFP) - The United States (US) is increasingly focused on how to secure Syria's chemical weapons if President Bashar al-Assad falls from power but is not considering sending ground troops into the war-torn country, Defence Secretary Leon Panetta said on Thursday.

While the US government has issued stern warnings to Damascus against resorting to such arms in its war with rebel forces, Mr Panetta said that a more likely scenario might be a chaotic vacuum if Mr Assad is toppled, with uncertainty over who controls the lethal weapons.

"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," Mr Panetta told a news conference. "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 a "hostile" setting.

"We're not talking about ground troops," Mr Panetta added.

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

Preventing the launch of chemical weapons "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," he said. "And that's unlikely, to be sure."

He said that clearly worded warnings to Mr 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.