"Retired" members of Iran's Revolutionary Guards and army are among 48 Iranians taken hostage in Syria by rebels on the weekend, Iranian Foreign Minister Ali Akbar Salehi said on Wednesday, the ISNA news agency reported.

But he denied the rebels' allegation that the Iranians had been on a military mission, saying the former military personnel had been exclusively on a religious pilgrimage to Damascus when they were seized on Saturday.

"A number of the (hostages) are retired members of the Guards and the army. Some others were from other ministries," Salehi was quoted as telling reporters as he flew back from Turkey, which he asked for help in freeing the Iranians.

Iranian officials had previously insisted the 48 Iranians were only pilgrims travelling to a holy Muslim site in Damascus. This was the first time Tehran admitted that any of them had a connection to its military.

Iran has given its full support to Syrian President Bashar al-Assad in his bloody fight against a nearly 17-month old insurgency, though it denies providing any military backing.

On Sunday, Syrian rebels posted a video of the hostages and claimed they were members of Iran's elite Revolutionary Guards on a "reconnaissance mission." They showed ID cards they said proved the military background of some of the Iranians.

Tehran denied the charge the Iranians were anything other than religious faithful on a private trip.

"Fortunately, in the video clip that was broadcast of these people, we see that these people are pilgrims and have no equipment other than clothes, personal items and identification cards," Salehi was quoted as saying.

He said the 48 abducted were in one of three buses taking Iranians to the airport in Damascus on Saturday. "There were three buses, two of which reached the airport," he said.

On its Facebook page, the rebel group on Monday said three of the Iranians had been killed in shelling by Syrian regime forces on their position. It threatened to execute other hostages if the bombardment did not stop.

But Iran's foreign ministry on Wednesday told Iran's Al-Alam television network that it believed "none of the hostages has been killed".

It was not possible to independently verify the fate of the hostages.

Iran on Tuesday sent a message to the United States through the Swiss embassy in Tehran saying that, "because of the United States' manifest support of terrorist groups and the dispatch of weapons to Syria, the United States is responsible for the lives of the 48 Iranian pilgrims abducted in Damascus."

The US State Department responded by saying Iran's position implicating it "doesn't seem to make sense." A spokesman, Patrick Ventrell, said the US government had no information about the hostages, their whereabouts, or their captors.

Iran on Tuesday dispatched Salehi to Turkey and Saeed Jalili, a top aide to supreme leader Ayatollah Ali Khamenei, to Syria to discuss the hostage situation.

Salehi also made a public appeal for their release.

"As we are in the sacred month of Ramadan, and both the hostages and kidnappers are Muslim, we send a message through the media to those who have taken our citizens: Take the necessary measures to release our citizens based on Islamic brotherhood," he said.

He also said that, in his meeting in Ankara with Turkish Foreign Minister Ahmet Davutoglu, he underlined that Iran was seeking inclusive dialogue on Syria to end the violence.

"I insisted that Iran is prepared to host a meeting with the (Syrian) opposition groups in Iran and wants them to sit next to each other in Iran and talk to the Syrian government," he said.

"We have talked with the Syrian government about this and it is ready as well."