Iran is helping to build and train a militia in Syria to prop up embattled President Bashar al-Assad, America's top military officer says.
Gen Martin Dempsey said the militia was intended to take the pressure off battle-weary Syrian regime forces.
Iran has described Syria as part of a vital regional alliance that Tehran will not allow to be broken.
Meanwhile, a summit of Islamic countries is due to suspend Syrian membership, despite Iranian objections.
The 57-member Organisation of Islamic Co-operation is expected to endorse the decision of its foreign ministers at the summit in Mecca, Saudi Arabia.
In an apparent conciliatory gesture, Saudi state TV showed King Abdullah welcoming leaders with Iranian President Mahmoud Ahmadinejad at his side. They were shown talking and laughing together.
Gen Dempsey, chairman of the US Joint Chiefs of Staff, said Syrian regime forces would be "taxed" after fighting for almost 18 months.
"They are having re-supply problems, they are having morale problems, they are having the kind of wear-and-tear that would come of being in a fight for as long as they have," he said.
He said Iran was training a militia made up of Syrian Shia Muslim fighters.
Syria's mainly Sunni Muslim rebels are being backed by Sunni-ruled Arab states such as Saudi Arabia and Qatar as well as Turkey.
Shia Iran supports President Assad, a member of the Alawite minority sect, which is an offshoot of Shia Islam.
Speaking at the same news conference, US Defence Secretary Leon Panetta said it had become obvious that Iran was providing assistance and training for the Syrian regime.
"We do not think that Iran ought to be playing that role at this moment in time," he said.
"It is adding to the killing that's going on in Syria, and it tries to bolster a regime that we think ultimately is going to come down."
Gen Dempsey said Washington had held talks with Syria's neighbours Jordan and Turkey about the possible need for a safe zone, amid an influx of refugees fleeing the fighting.
"With a safe haven would probably come some form of no-fly zone, but we are not planning anything unilaterally," he said.
Gen Dempsey also said it appeared that rebels had recently shot down a Syrian warplane, despite Damascus insisting that it had suffered a technical fault.
However, he said there was no indication that the rebels were armed with heavy weapons or surface-to-air missiles.
He said the jet could have been brought down with small arms fire.
Battle for Aleppo
Violence has spread in recent weeks to Syria's two biggest cities, Damascus and Aleppo.
The army is battling to regain control of Aleppo after retaking parts of Damascus seized by insurgents last month.
On Tuesday, activists in Aleppo said the army had shelled several rebel-held areas including Saif al-Dawla and Salah al-Din.
State news agency Sana reported that a number of "terrorists" had been killed or wounded in several areas of the city.
In Damascus, people were reportedly fleeing the central district of Qabun, fearing a military offensive.
The rebels have themselves have come in for increasing criticism because of their apparent mistreatment of prisoners in and around Aleppo.
A day after videos emerged of one man having his throat cut and the bodies of others being thrown off a roof, the Free Syrian Army (FSA) spoke out against what it termed "executions".
On Tuesday, former Syrian Prime Minister Riad Hijab, who defected to Jordan last week, said the Syrian government was collapsing "morally, financially and militarily".
Speaking in Amman, he said the regime controlled no more than 30% of Syrian territory.
He called on the opposition abroad to unite and on the Syrian army to stand alongside its people.
Valerie Amos, the UN under-secretary general and emergency relief co-ordinator, is currently in the region trying to improve the relief effort for refugees.
She is due to visit Lebanon where she will meet families who have fled Syria and will discuss with the government and humanitarian agencies how best to support those who have fled their homes.