WASHINGTON: The United States has largely ruled out sending in ground troops to secure Syrian chemical weapons under hostile circumstances, but the Pentagon could provide some forces if the Assad regime ever agrees to a peaceful transition, Defense Secretary Leon Panetta said Thursday.

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, acknowledged that it will be nearly impossible to prevent the Syrian government from using its chemical weapons, so the U.S. must rely on deterrence and continue warning Syria that using them would be unacceptable.

“The act of preventing the use of chemical weapons would be almost unachievable,” Dempsey said during a Pentagon press conference. “You would have to have such clarity of intelligence, you know, persistent surveillance, you’d have to actually see it before it happened, and that’s – that’s unlikely, to be sure.”

Speaking to Pentagon reporters, Panetta says his biggest concern is how the United States and allies would secure the chemical and biological weapons sites scattered across Syria and ensure the components don’t end up in the wrong hands if the regime falls, particularly under violent conditions. He said the U.S. is preparing no options for having U.S. ground troops in that country if the regime falls while under attack.

However, he added, “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.”

There are widespread worries among allies and countries in the region that if Syrian President Bashar Assad is toppled, Islamic extremists could gain control of Syria’s stockpile of chemical weapons, which includes sarin and mustard gas. And there are lingering worries that Assad might use his chemical weapons, perhaps on his own people, in a last-ditch effort to save his regime.

President Barack Obama has said the regime’s use of chemical weapons against the rebels would be a “red line” and change his “calculus” about possible military intervention there.

Fears escalated early last month when U.S. officials said there was evidence that Syrian forces had begun preparing sarin, a nerve agent, for possible use in bombs. But Panetta later said that it appeared the Syrian government had slowed its preparations for the possible use of the weapons.

The Pentagon has put together a variety of options for securing the weapons under a range of circumstances, Dempsey said. And he acknowledged the U.S. has been in contact with NATO allies, such as the Czech Republic, who have developed capabilities for handling chemical, biological and nuclear weapons. But Dempsey said no specific request has been made of the Czech Republic.

At least 60,000 people have died during Assad’s two-year crackdown on rebels, according to a recent U.N. estimate. Opposition fighters have seized large swaths of territory in northern Syria, and Thursday activists said they now control parts of a strategic air base. But despite significant rebel advances on the battlefield, the opposition remains outgunned by government forces and has been unable to break a stalemate on the ground.

Panetta Thursday said he believes there is a strong likelihood that Assad will ultimately leave power.