RENO, Nevada - US President Barack Obama has warned Syrian strongman Bashar al-Assad not to make the “tragic mistake” of deciding to unleash his stockpile of chemical weapons.

Assad’s beleaguered regime had earlier threatened to use such weapons if Syria faced international military intervention, although it vowed not to turn them against its own civilians. “Given the regime’s stockpile of chemical weapons, we will continue to make it clear to Assad and those around him that the world is watching,” Obama told an audience of US veterans in the western state of Nevada.

“They will be held accountable by the international community and the United States should they make the tragic mistake of using those weapons,” he said.

US officials also demanded that the Assad regime act responsibly and safeguard any such unconventional arms.

Denouncing Makdissi’s words as “horrific and chilling,” State Department spokeswoman Victoria Nuland said “that any possible use of these kinds of weapons would be completely unacceptable.”

“The Syrian regime has a responsibility to the world, has a responsibility first and foremost to its own citizens to protect and safeguard those weapons,” she insisted to reporters.

“That kind of loose talk just speaks to the kind of regime that we’re talking about.”

Pentagon press secretary George Little also warned Syria: “They should not think one iota about using chemical weapons.” Nuland said Washington was working with its allies to monitor the situation but refused to detail what kind of chemical weapons the Syrians might have in their arsenal, saying she could not discuss intelligence matters. Israel has also stepped up the rhetoric against Syria, warning it could take military action if any of its advanced weapons end up in the hands of Hezbollah. Nuland said the issue of Syria’s chemical weapons was among topics discussed earlier in July during US Secretary of State Hillary Clinton’s trip to Israel.

“Like all countries in the neighbourhood, it makes sense for there to be prudent planning for self-defence,” she said when asked about Israel’s threat of military action.

Meanwhile, Syrian rebels accused strongman Bashar al-Assad on Tuesday of moving chemical weapons to the country’s borders. The rebel Free Syrian Army said the regime’s chemical arsenal had been moved in a bid to pressure the international community, much of which has called for Assad to step aside in the face of the more than 16-month uprising against his rule.

“We in the joint command of the Free Syrian Army inside the country know very well the locations and positions of these weapons,” the statement said.

“We also reveal that Assad has transferred some of these weapons and equipment for mixing chemical components to airports on the border,” a statement said.

“According to our information, the regime began moving its stocks of weapons of mass destruction several months ago... with the goal of putting pressure on the region and the international community.”

More than 10,000 Iraqis have fled Syria to return home since Wednesday fearing for their lives, the UN refugee agency said. “Many of the returnees have expressed their fear regarding the ongoing risks to their safety in Iraq, but said they had little choice, given the security threats in Syria,” UNHCR spokeswoman Melissa Fleming said in Geneva on Tuesday.

The Iraqi government is bringing back returning Iraqi refugees by the planeload, she said, adding that the agency welcomed the announcement by Iraqi Prime Minister Nouri al Maliki that Syrians would continue to be allowed free access to cross into Iraq.

To date, some 7,500 Syrians have gone to Iraq, Fleming said, adding that in all nearly 150,000 Syrians were being sheltered in neighbouring countries, 30,000 more than on July 18.