BEIRUT — A suicide bomber detonated a truck filled with propane tanks at a crowded military checkpoint in central Syria on Sunday, killing more than 30 people, most of them civilians, in the second such attack by fighters linked to Al Qaeda in two days.
The attack, which was reported both by the state-run news media and antigovernment activists, occurred on a day when officials of the Arab League offered conflicting views on whether an international conference would be held next month on ways to halt the fighting.
The bombing shook the city of Hama, ignited dozens of cars, and sent up a column of smoke visible for miles. One activist said the secondary explosions of gas tanks continued long after the initial blast.
Activists said the Nusra Front, one of the two Al Qaeda affiliates fighting alongside the rebels who seek to topple President Bashar Assad, was responsible for the attack.
The bombing followed a similar attack east of Damascus the day before that killed 16 soldiers, suggesting an increasing reliance on suicide attacks to try to break government strongholds that the rebels are unable to take by conventional means.
But the high civilian toll Sunday worried antigovernment activists, who said it could lead to tensions between rebels and extremist allies.
The rise of extremist groups, which many rebels accept as battlefield companions even while disagreeing with their ideology, is one of the major challenges to international efforts to push for a negotiated end to the civil war. More than 100,000 people have been killed in Syria in 2½ years of conflict.
For months, the United States, Russia, the United Nations, and other major powers have been pushing for peace talks to be held in Geneva between the government and the opposition. But dates for the talks have been delayed repeatedly.
On Sunday, the secretary-general of the Arab League, Nabil al-Araby, suggested to reporters in Cairo that a conference would be held in Geneva on Nov. 23 and 24. But the joint UN-Arab League envoy for the Syria conflict, Lakhdar Brahimi, said at the same news conference that dates had yet to be set.
Brahimi said he would soon travel to Qatar and Turkey, which support the rebellion, and Iran, which backs Assad, as well as meet with US, Russian, and other international officials before official dates could be set.
So far, each side has insisted on conditions that the other side rejects. The government has said it will not negotiate with “terrorists,” while dismissing almost all who oppose it as such.
For its part, the opposition has demanded that the talks remove Assad from power. The government’s opponents are also hampered by internal divisions and by a widening gap between the exile political leadership, known as the Syrian National Coalition, and the hundreds of rebel groups fighting inside Syria, many of which have rejected the coalition.
Driving the wedge deeper between the two is the Nusra Front, which is shunned by the coalition but accepted by many rebels as a valuable ally against Assad’s forces.
Sunday’s attack, carried out by a Nusra bomber, struck a military checkpoint on the edge of Hama, next to a mechanization office belonging to the Syrian Ministry of Agriculture.
A local activist, Zakariya Attiya, said via Skype the military had turned the office into a base and ran a checkpoint on the road that searched all cars entering and leaving the city.
“That place was well known by all as the base from which they launched their operations in the area,” Attiya said. “They detain and torture people there.”
Attiya said that the Nusra Front had carried out the attack because the rebels, who have taken over towns in the surrounding countryside, lack the military might to challenge the government’s grip on the city itself. Although other rebels in the area do not use suicide bombers, they do not complain if Nusra does, as long as they target the government.
But the high civilian toll disturbed many in the opposition.
The casualty estimates varied. The Syrian Observatory for Human Rights said that 43 people were killed, at least 32 of them civilians.
The group supports the opposition and tracks the conflict from Britain through a network of contacts on the ground.
Syria’s state news agency, SANA, reported the death toll as 37 but did not say that any soldiers had been killed.
The Syrian National Coalition, the opposition umbrella group that has Western support, is scheduled to meet Nov. 1-2 in Istanbul to decide whether to take part in the proposed Geneva conference.
A prominent faction within the coalition, the Syrian National Council, has said it has no faith in talks with Assad’s regime and will not attend any Geneva negotiations.
And the coalition’s ability to speak for the broader rebellion has long been in dispute. Fighters inside Syria — many of whom reject negotiations with the regime — have accused the opposition leaders in exile of being out of touch with reality on the ground.
The credibility of the coalition took a major hit last month when nearly a dozen prominent rebel groups publicly broke with the opposition umbrella group. More rebel brigades have since followed suit.
The government, meanwhile, has kept its options open on the Geneva conference.
Some officials have said all opposition groups should be represented in the talks, while others have refused to deal with the coalition.
Assad, however, has stuck to one point throughout: a refusal to talk with ‘‘terrorists,’’ the term the government uses for those trying to topple the president by force.