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.
Saudi state TV showed King Abdullah welcoming leaders with Iranian President Mahmoud Ahmadinejad at his side. They were shown talking and laughing together.
'Bolstering doomed regime'
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.
Iran-Syria strategic alliance
After Iran's 1979 revolution, Damascus and Tehran opposed Saddam Hussein's Iraq
Both aim to check Israeli advances in Lebanon and to prevent US influence in Middle East
They support the Lebanese Shia movement Hezbollah and Palestinian militant group Hamas
In August 2012, Iran declared Syria an essential part of an "axis of resistance"
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.
In other developments
President Assad's special envoy, Bouthaina Shaaban, is in Beijing for talks with Chinese Foreign Minister Yang Jiechi
Communist Party mouthpiece People's Daily accuses some Western countries of seeking regime change in Syria
UN humanitarian chief Valerie Amos is due to visit Lebanon during a regional tour to improve aid for Syrian refugees
Ex-Syrian PM Riad Hijab, who defected to Jordan last week, said the Syrian government was collapsing "morally, financially and militarily"
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".