DAMASCUS - Syrian peace talks can only be held if a "credible opposition" takes part, an international envoy said Sunday, as a truck bombing in the war-ravaged country killed dozens.

Arab League chief Nabil al-Arabi told reporters in Cairo the talks would be held on November 23, but UN-Arab League envoy Lakhdar Brahimi, who spoke at the same news conference, refused to set a date.

"It was decided that the Geneva 2 conference will be held on November 23, and preparations are underway for this conference," said Arabi.

Brahimi cautioned the meeting would only go ahead in the presence of a "credible opposition representing an important segment of the Syrian people" opposed to President Bashar al-Assad.

"There is agreement to hold Geneva 2 in November, but the date has not been officially set... We hope the conference will be held in November."

Brahimi is on the first leg of a Middle East tour aimed at drumming up support for the initiative to end the 31-month conflict that has killed more than 115,000 people.

The veteran troubleshooter has said he would also travel to Qatar, Turkey, Iran, Syria and then Geneva for talks with Russian and US representatives.

Al-Watan, a pro-Damascus newspaper, said Brahimi would visit Syria next week.

Washington and Moscow have been trying to organise the conference on the heels of a landmark deal they reached for Syria to destroy its chemical weapons by mid-2014.

The Geneva initiative was first announced last year, but it has been repeatedly postponed amid opposition wrangling and a dispute over which countries, including Iran, should participate.

Syria has heavily criticised the envoy, especially after he suggested a transitional government be set up and given full powers until elections, following his last visit in late 2012. Al-Watan said Damascus was ready to welcome him as long as "he works as a mediator, not as a party in the international conflict over Syria".

But Syria has consistently refused to enter negotiations that demand Assad quit power as a condition.

Meanwhile, the National Coalition umbrella opposition group said its members would decide in the coming days whether to attend the Geneva talks, while the Syrian National Council, a key component of the bloc, has threatened to quit if they do.

But even if the Coalition attends the Geneva meeting, it is unclear whether it can enforce any agreement, after dozens of rebel brigades have in recent weeks rejected the umbrella group.

On the ground, a truck bomb killed at least 31 people in Hama, a regime-held city in central Syria, activists said.

"At least 31 people, including regime troops, were killed when a man detonated a truck laden with explosives at a checkpoint near an agricultural vehicles company on the road linking Hama to Salamiyeh," the Syrian Observatory for Human Rights said, adding the toll was expected to rise.

State media said 30 people died.

Hama is the site of a 1982 massacre of some 10,000 to 40,000 people ordered by president Hafez al-Assad during a crackdown on an earlier revolt.

The attack came a day after rebels from the Al-Nusra Front, another Al-Qaeda-linked group, set off a car bomb and launched a major assault on a checkpoint near the mixed Christian-Druze neighbourhood of Jaramana in Damascus.

UN humanitarian chief Valerie Amos has called for a ceasefire in another embattled Damascus suburb, Moadamiyet al-Sham, where thousands of people "remain trapped".

The southwestern district was one of a number of suburbs hit in an August 21 sarin gas attack, blamed by the opposition on the regime, which led to the deal to dismantle Syria's chemical arsenal.

In the north, the air force carried out new strikes on rebels around Aleppo central prison, which they are trying to wrest from government control, said the Observatory.

Meanwhile, nine Lebanese Shiite pilgrims seized by Syrian rebels 17 months ago and two Turkish pilots kidnapped in Beirut in August arrived back home under an exchange deal mediated by Turkey and Qatar.

The rebels had also demanded the release of scores of women prisoners from Syrian jails in exchange for freeing the pilgrims. It was unclear if those releases went ahead.