A top Iranian military chief warned on Sunday that the US will face "harsh consequences" if it intervenes in ally Syria over claims of chemical attacks, Fars news agency reported.
"If the United States crosses this red line, there will be harsh consequences for the White House," armed forces deputy chief of staff Massoud Jazayeri was quoted as saying.
A year ago US President Barack Obama warned the use of chemical weapons in Syria would cross a "red line" and have "enormous consequences".
On Sunday, his Defence Secretary Chuck Hagel said the US military was ready to take action against Syria.
"President Obama has asked the Defence Department to prepare options for all contingencies. We have done that," Hagel told reporters in Malaysia.
"Again, we are prepared to exercise whatever option, if he decides to employ one of those options," he said, a day after Obama held a rare meeting his top aides and brass to discuss Syria.
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After the meeting, Obama spoke by phone with Prime Minister David Cameron of Britain, which has accused the Syrian government of using chemical weapons on a large scale.
A statement from Cameron's office said if the use of chemical weapons by Syria would "merit a serious response" -- echoing French calls that "force" be used if the claims are confirmed.
But the Iranian military leader warned Washington, its Western allies and Israel against playing with "fire".
"The terrorist war under way in Syria was planned by the United States and reactionary countries in the region against the resistance front (against Israel)," Fars quoted Jazayeri as saying.
"Despite this, the government and people of Syria have achieved huge successes.
"Those who add fire to the oil will not escape the vengeance of the people," added Jazayeri.
Foes of the Damascus regime say Syrian forces unleashed a chemical attack Wednesday on areas southwest and east of the capital, killing hundreds of people.
The regime of President Bashar al-Assad denies the accusations, saying instead that rebel fighters have used chemical weapons in the 29-month conflict.