"For Jordan, the more unstable Syria becomes, the deeper the crisis proceeds, the more likely Jordan will suffer from all kinds of spillover, but they are incapable of doing anything to intervene to try to turn the conflict in one direction rather than another unless they have the ballast, cover and involvement of serious international forces, which is the Americans," Hollis said.
She also saw the American military presence as a step toward possible future military operations to secure Syria's chemical stockpiles.
Torbjorn Soltvedt, a senior analyst with the Britain-based Maplecroft risk analysis group, said he saw the current situation as a "monitoring and training stage."
"Given the degree to which Syria's chemical weapons stockpiles have been dispersed across the country, an operation to secure them would be extensive and require significant numbers of troops," he said. "The Pentagon has estimated that an operation to secure Syria's chemical weapons stockpiles could require as much as 75,000 troops given the presence of several chemical agent manufacturing plants and many more storage sites throughout the country."
Panetta said that while the U.S. believes the weapons are still secure, intelligence suggests the regime might have moved some to protect them.
Steven Bucci, an expert in chemical weapons at the Heritage Foundation, has told Congress there might be as many as 50 chemical weapons sites in Syria. 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.
At the desert facility, stretching 25 kilometers (16 miles) on the edge of this predominantly Palestinian suburb, Jordanian soldiers guard the walled compound, where Iraqi and Libyan special forces once received training. They refused to allow reporters in.
Jordanian officials were eager to downplay the U.S. role, concerned about the possibility of raising tensions with Syria and giving the kingdom's largely conservative population the impression that they were allowing foreigners to use Jordan as a potential launching pad for a pre-emptive attack against another Arab country.
The senior government official and two others who discussed the American military role all spoke on condition of anonymity, citing possible diplomatic sensitivities with Syria. Assad is thought to have sleeper cells scattered across the kingdom and plotting attacks on Syrian opposition and Jordanian figures.
Information Minister Sameeh Maaytah, the only official who spoke on the record, said the U.S. presence was part of "routine training exercises."
"Jordan and U.S. forces exchange visits regularly, and the presence of tens of their forces here is part of efforts to expand cooperation, exchange capabilities and protect regional stability," he said in an interview. He declined to elaborate or comment on any link to the Syrian crisis.
Amman has long had bumpy relations with Damascus because of its alliance with the United States — Jordan's largest donor of economic and military aid — and its 1994 peace treaty with Israel.
Jordan would like to see the Syrian regime toppled because of growing concern that Assad's key ally, Iran, is trying to spark Shiite uprisings in Arab countries ruled by members of the rival Sunni sect. Assad's ruling Alawite minority is an offshoot of Shiite Islam.
Abdullah was the first Arab leader to warn in 2004 of the sweep of Iran's "Shiite crescent," stretching from Lebanon through Syria and Iraq.
Jordanian officials have advocated a buffer zone inside the Syrian border to protect civilians fleeing bombardment. There is mounting speculation that Jordan would dispatch highly skilled special forces to secure such a zone when Assad's regime falls to prevent chaos on its border.
In the past six weeks, more than 20 Syrian rockets have fallen on Jordanian villages near the border. At least two people were wounded, including a 4-year-old Jordanian girl.
The two Western diplomats said the Britons, about a half-dozen officers specialized in intelligence gathering and special operations techniques, visited Jordan a few times over the past three months. The diplomats insisted on anonymity, saying that public comment may hamper their information gathering on Syria.
The Jordanian army already has an extensive presence on the border and has been assisting waves of Syrian refugees, who are straining the country's meager resources, mainly health care, water and utilities.
Jordan hosts some 200,000 Syrian refugees, more than any other neighboring country. Some come under constant firing from their army as they cross into the kingdom. Jordanian border guards have been wounded and a 6-year-old Syrian boy was killed in July.
Jordanian men also are moving the other way across the border, joining what intelligence officials have estimated to be about 2,000 foreigners fighting alongside Syrian rebels trying to topple Assad.
Associated Press writer Dale Gavlak in Amman, Jordan, contributed to this report.
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