The United States on Tuesday accused Iran of setting up a pro-regime militia in Syria as Washington increasingly ties the crisis there to interference by its long-time foe Tehran.
Western powers are already locked in a diplomatic stand-off with Iran over what they say is its plan to develop nuclear weapons, and tensions are high between US and Iranian patrols in the oil shipping lanes of the Gulf.
Iran and Syria are long-standing allies and joint sponsors of the Lebanese armed group Hezbollah, but the formation of an Iranian-backed force inside Syria would mark a new and dangerous chapter in an already bloody conflict.
Rights groups say more than 23,000 people have been killed in 17 months of fighting between troops loyal to Syrian President Bashar al-Assad and rebel forces, including civilians killed in the regime's brutal crackdown.
"It is obvious that Iran has been playing a larger role in Syria in many ways," US Defense Secretary Leon Panetta said at a joint press conference with the chairman of the Joint Chiefs of Staff, General Martin Dempsey.
Panetta said the United States has evidence that Iran's Revolutionary Guards are "trying to develop, trying to train a militia within Syria to be able to fight on behalf of the regime."
"We are seeing a growing presence by Iran and that is of deep concern to us. We do not think that Iran ought to play that role at this moment in time, that's dangerous... It's adding to the killing that's going on in Syria."
Panetta urged Tehran to stay out of the conflict, saying: "Our hope is that Iran thinks better about how much they do want to get involved. The Syrian people ought to determine their future, not Iran."
Last week, Syrian rebels captured 48 Iranians they accused of supporting the regime. Tehran admitted that some of the men were "retired" Revolutionary Guards, but insisted they were on a religious pilgrimage.
The Revolutionary Guards, an elite military organization, has a history of sponsoring armed groups abroad in order to further Iran's interests.
Apart from Hezbollah, a powerful Shiite armed faction in Lebanon, Iran has also been accused of supporting various Shiite militias in Iraq, such as elements of the Mahdi Army of firebrand cleric Moqtada al-Sadr.
Hezbollah has proved one of Israel's deadliest foes in clashes in southern Lebanon and stands accused of carrying out bomb attacks around the world. The Mahdi Army at times bitterly resisted the former US occupation of Iraq.
The US administration had already accused Hezbollah of lending aid to Assad's embattled regime, and Dempsey said the new militia is based on the model of the Mahdi Army and has Shiite and Alawite recruits.
Assad's regime is dominated by figures from his Alawite minority, a sub-branch of Shiite Islam, and the defections of several senior Sunni Muslim officials in recent weeks has sharpened the country's sectarian divide.
The bulk of the rebels are from Syria's Sunni majority, and both sides have been accused of attacks on members of rival communities.
The US officials said Syria's army has been weakened by desertions and defections within its top hierarchy.
"The Syrian army has been fighting now for about 18 months or so. And any army would be taxed with that kind of pace," Dempsey said, adding he expected that the Syrian military was experiencing resupply and morale problems.
"That's why Iran is stepping in to form this militia, to take some of the pressure off of the Syrian military."
Panetta said that Washington would continue to provide humanitarian aid to Syria refugees, to monitor the regime's chemical weapons stockpile and to send "non-lethal" supplies to the rebels.
He added that some of the United States' regional allies would provide "more aggressive assistance to the opposition as well" -- an apparent reference to reported arms shipments from Turkey, Qatar and Saudi Arabia.