Speaking at a NATO conference of defense ministers, Panetta said the U.S. has been working with Jordan to monitor chemical and biological weapons sites in Syria and also to help Jordan deal with refugees pouring over the border from Syria.
About 150 U.S. troops, largely Army special operations forces, are working out of a military center near Amman, two senior defense officials said on condition of anonymity because they were not authorized to speak publicly about the mission. The troops have moved back and forth to the Syrian border as part of their work, which is joint planning and intelligence gathering, one official said.
The revelation of U.S. military personnel so close to the 19-month-old Syrian conflict suggests an escalation in the U.S. involvement in the conflict, even as the Obama administration pushes back on any suggestion of a direct intervention in Syria.
News of the U.S. mission to Jordan also follows several days of shelling between Turkey and Syria, an indication that the civil war could become a regional conflict. One of the U.S. defense officials said the extra planning is aimed at avoiding those kinds of clashes between Jordan and Syria.
The development comes with the U.S. presidential election less than a month away, as Republican nominee Mitt Romney criticizes President Barack Obama for weak leadership in foreign policy. Romney has said he would send U.S. troops into Syria if needed to prevent the spread of chemical weapons, while Obama has said that movement or use of chemical weapons would have "enormous consequences."
Panetta has said that while the U.S. believes the weapons are still secure, intelligence suggests the regime might have moved some to protect them.
Syria is believed to have one of the world's largest chemical weapons programs, and the Assad regime has said it might use the weapons against external threats, though not against Syrians. The U.S. and Jordan share the same concern about Syria's chemical and biological weapons - that they could fall into the wrong hands should the regime in Syria collapse and lose control of them.
Jordan's King Abdullah II fears such weapons could go to the al-Qaida terror network or other militants, primarily the Iranian-allied Lebanese Hezbollah - a vocal critic of Jordan's longstanding alliance with the United States.
The Monterey, Calif.-based James Martin Center for Nonproliferation Studies provided a map purporting to show four Syrian production sites for chemical weapons, three for storage, one for research and development, and two with dual use infrastructure.
Steven Bucci, an expert in chemical weapons at the Heritage Foundation, has told Congress there might be as many as 50 chemical weapons sites. He said in an interview Wednesday that Syria's stockpile is potentially "like a gift from God" for militants since they don't have the know-how to assemble such weapons, while some of Syria's chemical agents are believed to have already been fitted into missile warheads.
Pentagon press secretary George Little, traveling with Panetta, said the U.S. and Jordan agreed that "increased cooperation and more detailed planning are necessary in order to respond to the severe consequences of the Assad regime's brutality."
He said the U.S. has provided medical kits, water tanks and other forms of humanitarian aid to help Jordanians assist Syrian refugees fleeing into their country.
"We have a group of our forces there working to help build a headquarters there and to insure that we make the relationship between the United States and Jordan a strong one so that we can deal with all the possible consequences of what's happening in Syria," Panetta said.
In Jordan, the biggest problem at the moment seems to be the strain put on the country's meager resources by the estimated 200,000 Syrian refugees who have flooded across the border - the largest number fleeing to any country.
Several dozen refugees in Jordan rioted in their desert border camp of Zaatari earlier this month, destroying tents and medicine and leaving scores of refugee families out in the night cold.
Jordanian men also are moving the other way across the border, joining what intelligence officials have estimated to be around 2,000 foreigners fighting alongside Syrian rebels trying to topple Assad. A Jordanian border guard was wounded after armed men - believed trying to go fight - exchanged gunfire at the northern frontier.
Turkey has reinforced its border with artillery and deployed more fighter jets to an air base close to the border region after an errant Syrian mortar shell killed five people in a Turkish border town last week and Turkey retaliated with artillery strikes.
Turkey's military chief, Gen. Necdet Ozel, vowed Wednesday to respond with more force to any further shelling from Syria, keeping up the pressure on its southern neighbor a day after NATO said it stood ready to defend Turkey.
Gen. Martin Dempsey, the chairman of the U.S. Joint Chiefs of Staff, said in Washington on Wednesday that the Pentagon was planning for "a number of contingencies" and was prepared to provide the administration with options on Syria, if needed.
"But the military instrument of power at this point is not the prominent instrument of power that should be applied in Syria," he said.