Iran has been accused by the US administration of setting up pro-government militias in Syria.
Leon Panetta, US defence secretary, said on Tuesday that Iran is giving the fighters training and weapons.
"It is obvious that Iran has been playing a larger role in Syria in many ways," he said at a joint press conference in Washington, DC with the chairman of the joint chiefs of staff, General Martin Dempsey.
Panetta said Iran's growing presence could only aggravate the situation on the ground.
In-depth coverage of escalating violence across Syria
There is now 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", Panetta said.
"So 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."
Violence continued inside Syria on Wednesday, including a bomb explosion near the hotel used by the UN observer mission in central Damascus.
Most of the members of the alleged militia are Shia, and some are Alawites like Syrian President Bashar al-Assad, Dempsey said, comparing the force to the Mahdi Army of the Iraqi Shia leader Muqtada al-Sadr, an outspoken critic of the US invasion of his homeland.
"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, noting he expected that the Syrian military was experiencing resupply and morale problems.
Dempsey also said it appears that Syrian rebels are able to shoot down a Syrian warplane, but said he has seen no indication that they are armed with heavy weapons or surface-to-air missiles.
He says the MiG fighter could have been shot down with small-arms fire.
Syria has blamed the crash on a technical malfunction.
"The Syrian people ought to determine their future, not Iran," Panetta said.
Ghanbar Naderi, an Iranian journalist, told with Al Jazeera that this type of allegation from the US and its allies was not new.
"It's obvious that armed groups and gangs have been unable to topple Bashar al-Assad after more than a year, and now they are trying to find a scape goat in Iran again," he said in an interview from Tehran.
"This is not just about Syria, this is about finding an excuse to attack Iran."
The US has itself faced criticism of intervention from Iran, which has criticised Qatar, Saudi Arabia and Turkey of arming the opposition in Syria in collusion with the US and Israel.
On August 4, Ahmad Vahidi, Iran's defence minister, said that "the region will face a major crisis if foreign forces, currently [covertly] present in Syria, enter the scene" and intervene militarily.
"The losers of such a crisis will be the Westerners and pro-Zionist countries," he predicted in comments report on Iranian television.
"It is ugly that countries ... have flooded Syria with arms, equipping terrorist groups. It will have very bad implications in the region."
Ban Ki-moon, UN secretary-general, has called the Syrian conflict a "proxy war, with regional and international players arming one side or the other".
Valerie Amos, the United Nations humanitarian chief who is currently visiting Syria, told Al Jazeera on Wednesday that "the responsibility [for the humanitarian suffering of so many civilians] rests with all of those involved with the fighting".
Panetta's comments come as an emergency summit of the 57-member Organisation of Islamic Co-operation (OIC) got under way in the western Saudi city of Jeddah.
The summit opened late on Tuesday with a proposal to suspend Syria, a move strongly opposed by Iran.
A draft final statement obtained by AFP said the summit "approves the suspension of Syria's membership", a measure recommended by a preparatory ministerial meeting held on Monday.
Iranian President Mahmoud Ahmadinejad, whose country has openly criticised the push to suspend Syria, is attending the extraordinary meeting and was expected to strongly oppose the move to suspend Syria.
Ali Akbar Salehi, the Iranian foreign minister, criticised the move to suspend Syria's membership of the OIC, saying it would not resolve the conflict and was not in line with the group's charter.
"We have to look for other ways, means and mechanisms for resolving conflicts and crises," he said on Monday, calling for a "Syrian-Syrian solution" reached through negotiations between the government and the rebels.