The US military's top officer declined Thursday to divulge his advice to President Barack Obama on whether America should take military action in Syria, despite complaints from some lawmakers.

General Martin Dempsey, chairman of the Joint Chiefs of Staff, told senators the Obama administration was weighing possible intervention against the Syrian regime based on unspecified "options" provided by the military.

"That issue is under deliberation inside of ... our agencies of government, and it would be inappropriate for me to try to influence the decision with me rendering an opinion in public about what kind of force we should use," he said at a Senate Armed Services Committee hearing.

His answer frustrated some lawmakers including Senator John McCain, an outspoken advocate for armed action in Syria who rebuked Dempsey for not providing the panel with his opinion.

"This goes back to my concern about your role as chairman of the Joint Chiefs," said McCain, saying it was the general's duty to offer his candid views to lawmakers.

Dempsey, who is up for re-appointment for another two-year term, has often cited the potential risks of intervening against President Bashar al-Assad's regime.

But the four-star general denied Thursday he was trying to block US action in Syria.

"Senator, somehow you've got me portrayed as the -- you know, the one who's holding back from our use of military force inside of Syria," Dempsey said.

"Members of this committee have been briefed on them (military options) in a classified setting. We've articulated the risk," he added.

"The decision to use force is the decision of our elected officials."

McCain grilled Dempsey about his evolving views on arming the Syrian rebels.

He recounted how the general in February said he supported the move, opposed it in April and now apparently again backs the idea, following a recent decision by Obama to supply weapons to the Syrian opposition.

"How do we account for those pirouettes?" McCain asked.

Dempsey rejected the description and said there had been concerns earlier this year about the influence of extremist groups in the opposition.

"I support the building of a moderate opposition, and including building its military capability," he said.

McCain repeatedly asked if US "inaction" could carry a higher cost than intervention but Dempsey insisted Washington had not been inactive.

And he fired back his own question at McCain that seemed to invoke the miscalculations of the US war in Iraq.

"Senator, would you agree that we have recent experience where until we understood how the country would continue to govern and that institutions of governance wouldn't fail, that actually, situations can be made worse by the introduction of military force?"

The tense exchange came amid reports the Assad regime has gained momentum in its fight with rebel forces and after top US diplomat John Kerry paid a visit to a Syrian camp in Jordan, where he met angry refugees who pleaded for Washington to set up a no-fly zone.