Syrian state TV says a suicide car bombing outside a bus station near Damascus has killed at least 15 people.

State television says Tuesday's attack took place in the Sumariyah area southwest of the capital, wounding more than 30.

The Britain-based Syrian Observatory for Human Rights also put the death toll at 15. It said nine civilians and six government troops were killed.

No further details were immediately available.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

Syria's main opposition group in exile said Tuesday it had not made a final decision yet on whether it would participate in a peace conference to be held in Geneva, adding that the international community must "prove its seriousness" by forcing Damascus to agree on trust-building measures.

The comments illustrate the huge hurdles that lie ahead in bringing the warring sides together, even as world powers backing the conference announced Jan. 22 as the date for the first face-to-face talks.

"We did not make a final decision yet on our participation in the Geneva conference," said Ahmad al-Jarba, chief of the Syrian National Coalition. He said his group has indicated its desire to go, "but we think that the Syrian regime is the one which doesn't want to go to Geneva 2 but the Russians are putting pressure on them to attend."

The opposition has previously argued that there is no use talking to the government unless it could lead to President Bashar Assad stepping down, and that negotiations for their own sake would merely prolong the conflict.

Al-Jarba spoke to reporters in Cairo early Tuesday, where he is scheduled to meet with senior Egyptian and Arab League officials.

Previous attempts to bring the two sides together have failed, mainly because of disputes over who should represent the opposition and the government, Assad's future role in the country, and whether Iran, Saudi Arabia and other regional powers should be at the table.

The U.N. set the date on Monday.

U.N. Secretary-General Ban Ki-moon urged the government and opposition to help the Jan. 22 conference to be held in Geneva succeed by taking steps to stop the violence, provide access for desperately needed humanitarian aid, release detainees, and help hundreds of thousands of refugees and internally displaced people return to their homes.

"We go with a clear understanding: The Geneva conference is the vehicle for a peaceful transition that fulfills the legitimate aspirations of all the Syrian people for freedom and dignity, and which guarantees safety and protection to all communities in Syria," he told reporters at U.N. headquarters.

He said a key goal would be the establishment of a transitional government with powers over the military and security forces. A full list of participants has not yet been decided on.

The coalition, in a statement issued Tuesday, reiterated its stand that there can be no role for Assad in any future transitional government.

"Bashar Assad or any of the criminals responsible for killing the Syrian people cannot be part of any transitional body and cannot have any role in Syria's political future," the statement said.

It also urges the international community to pressure Assad's government to secure humanitarian corridors so that aid can reach besieged opposition held areas of the country.

The statement came as the World Health Organization said it has discovered two additional cases of the highly contagious polio virus in Syria, bringing the total number of cases in the country to 17.

Sona Bari, an official with the U.N. agency said the new cases are in Aleppo and in rural Damascus _ far from where the first outbreak was discovered in the northeast province of Deir el-Zour in early November.

Those cases were the first discovered in Syria for over a decade, caused by the collapse of the health system in conflict areas. Half a million children were left without vaccinations.

Following the outbreak, the U.N. began its largest polio vaccination campaign in the region's history, trying to vaccinate and re-vaccinate 20 million children from Iraq to Turkey.