The United States is not on the brink of military action despite an assessment from US spy agencies that the Syrian regime likely used chemical agents, a US defense official said Thursday.

The US military has prepared contingency plans for the Syrian conflict, but officials traveling with Defense Secretary Chuck Hagel in the Middle East suggested military action was not a certainty -- at least for the moment.

"It's our job... to present options to the president upon request," the senior defence official, who spoke on condition of anonymity, told reporters in Abu Dhabi.

But the official added: "Intelligence assessments don't automatically trigger policy decisions. It's important to note in this case."

Alluding to the disastrous intelligence failure in the run-up to the 2003 US invasion of Iraq, the official indicated the White House would be careful not to allow intelligence reports to drive a decision to use military force.

Former officials and analysts say the United States could try to secure Syria's chemical arsenal by sending in special forces teams and launching bombing raids.

But any military action would be high-risk with a chance that weapons could fall into the hands of extremists.

Deploying troops would require warplanes to knock out Syria's air defenses first, allowing special forces to be flown in, experts say.

The Pentagon has already sent more than 200 troops to Jordan, including a US Army headquarters element, to prepare for a possible joint operation with allies to secure chemical weapons.

US intervention no longer appeared as a remote possibility after Hagel and the White House said Thursday that American spy agencies had concluded President Bashar al-Assad's regime probably had fired deadly sarin gas against rebel forces on a "small scale."

In a letter informing members of Congress of the findings, the White House said the intelligence services had "varying degrees of confidence" that the regime had drawn on its chemical arsenal.

The defence official confirmed that the phrase "varying degrees of confidence" is a term commonly used by the intelligence community to indicate disagreement among various agencies.

But the assessment reflected a degree of certainty that Syria most likely has fired chemical agents and was not merely a tentative suspicion, the official said.

"There are very strong indications at this point that chemical weapons have been used in Syria," the official said.

Although British, French and Israeli officials had for weeks privately pointed to mounting evidence that Syria was resorting to chemical agents, the official rejected criticism that US spy services may have lagged behind other countries.

"The United States reaches conclusions on intelligence on our time frame. And is not driven by other countries."

The official spoke to reporters accompanying Hagel on his first trip to the Middle East as Pentagon chief.

Concerns over Syria's civil war and fears that chemical weapons had been unleashed dominated Hagel's week-long tour, that included stops in Israel, Jordan, Saudi Arabia, Egypt and the United Arab Emirates.