A pan-Arab television station aired a video Sunday purporting to show Syrian rebels guarding a group of Iranians abducted a day earlier and promising more attacks on Iranian targets.

Armed men in the video identify themselves as members of the rebel "Baraa Brigades" and say that at least one of the 48 captives was an officer of Iran's powerful Revolutionary Guards.

Syrian rebels claim the Iranians were on a "reconnaissance mission" in the capital Damascus at the time they were abducted.

Iran says they are pilgrims who were visiting a shrine.

This video image released by the Baraa Brigades purports to show Free Syrian Army soldiers guarding a group of Iranians abducted on Saturday. (Baraa Brigades/Associated Press)

"We promise Iran and all those who support this regime ... we will strike at all (Iranian) targets in Syria," one of the rebels says in the video. "The fate of all Iranians who operate in Syria will be the same as those we have here, either captive or killed, God willing."

The abductions threaten to pull Syria's close ally Iran deeper into the country's civil war. They also raised questions about the extent to which President Bashar al-Assad's regime can control the centre of its power in the capital.

Last month, rebels and Syrian regime forces fought intense battles for a week in Damascus, the opposition fighters' biggest challenge so far in the capital.

Government claims control over Damascus

The government claimed Saturday it was now in full control of all districts in Damascus after purging Tadamon, one of the last rebel-held areas.

But several residents reported hearing loud explosions and gunfire echoing from several districts of the capital throughout the night and early Sunday.

Activists and residents said there were clashes in the central districts of Rukneddine, Salhiyeh and al-Muhajireen on the foothills of Mount Qassioun.

And in a rare acknowledgement of an attack in the heart of Damascus, state-run news agency SANA said a Syrian was killed and several others were wounded when "terrorist groups" fired mortar shells behind the Abbassiyyin hospital, a few hundred metres from Abbassiyyin Square, a major roundabout in Damascus.

Syrian authorities regularly use the word "terrorists" to refer to the rebels seeking to topple the regime. Khaled al-Shami, an activist in Damascus, said official reports that troops had purged Damascus from rebels were "nonsense."

"The Free Syrian Army does not seek to hold territory in Damascus but rather stage hit and run attacks that drain the regime. The rebels are present and strong there," he said.

Gunmen snatched 48 Iranian pilgrims just outside Damascus on Saturday in a brazen attack.

The pilgrims were on a bus taking them from the suburb of Sayeda Zeinab, about 16 kilometres south of Damascus, to the airport to return home when they were kidnapped, according to the Iranian state news agency, IRNA.

Mainly Shia Iran is a close ally of the beleaguered Syrian government, which is dominated by the minority Alawite sect, an offshoot of Shia Islam.

Rebels say Iranian officer was among pilgrims

Syria has long welcomed Iranian pilgrims visiting the ornate, gold-domed shrine of Sayeda Zeinab, the Prophet Muhammad's granddaughter. But the rebels in the video claimed there was an officer in Iran's Revolutionary Guard among the group and showed what was purportedly his ID and a permit to carry weapons.

IRNA said Sunday that Iranian Foreign Minister Ali Akbar Salehi had spoken by phone with his Turkish and Qatari counterparts and demanded their intervention to help release the Iranians. Turkey and Qatar have supported the overwhelmingly Sunni Muslim rebels in Syria.

The Turkish and Qatari foreign ministers promised to make efforts for the release of Iranian pilgrims, IRNA reported Sunday. In Tehran, a senior member of an influential parliamentary committee advised Iranians against travelling to Syria, state-run Press TV reported, in a high-ranking acknowledgment that Syrian rebels have expanded their hold over key roads and other areas once firmly under Assad's control.

The comment by Kazem Jalali, a member of Iran's Committee on Security and Foreign Policy, does not represent an official Iranian discouraging travel to Syria, but the views of the parliament group often shape policies.

Hillary Clinton to visit Turkey

U.S. Secretary of State Hillary Clinton will visit Istanbul later this week for talks with Turkish officials over the worsening crisis in Syria, the State Department said Sunday.

Turkey, which has taken in tens of thousands of Syrian refugees, has beefed up its military presence along the border.

The main battle in the civil war has now shifted to Syria's largest city, the commercial hub of Aleppo, some 350 kilometres north of Damascus. Rebels seized several neighbourhoods there two weeks ago and the regime has struggled to dislodge them ever since in a stark demonstration of the rebels' growing strength and organization and the regime's loosening grip on the country.

On Sunday, Syrian opposition groups and activists said fierce clashes were still ongoing as rebels tried to expand their hold and inch closer to the historic city centre.

Local activist Mohammad Saeed said there is fighting only a few hundred metres from the medieval citadel overlooking the city centre.

Heavily armed government troops have been steadily shelling rebel-controlled districts, mainly in the southwestern part of the city, for the past two weeks.

Saeed and the British-based Syrian Observatory for Human Rights reported heavy shelling and clashes Sunday mainly in the districts of Salaheddine, al-Sukkari and Hananou.

The group said troops were using fighter jets to try and crush the rebels, mostly strafing but also bombing.

"Fighter jets to us are now as common as birds in the sky," Saeed said. Activists have been saying for a week that the government is gearing up for an all-out offensive to clear Aleppo from rebels, and the pro-government Al Watan newspaper said Sunday that the Syrian army is bracing itself for the "decisive battle."

But expectations of an imminent assault have not yet materialized.