The Britain-based Syrian Observatory for Human Rights says the attack in the village of Ziyara in the central province of Hama was carried out by Jabhat al-Nusra, an al-Qaida-inspired Islamic militant group.
The death toll could not be independently verified.
State-run news agency SANA said the blast occurred outside a state-run development agency. It said Monday's blast caused deaths but gave no figure.
The Observatory also reported that an air raid on the northern town of Harem killed at least 20 rebels. It said a rebel commander was seriously wounded in the raid.
Rebels and government troops backed by fighters from a Palestinian radical group fought in the capital Damascus on Monday in some of the most intense fighting in the city in nearly three months, activists said.
In northern Syria, rare fighting broke out between rival rebel groups for control of a border crossing with Turkey, according to an opposition figure.
The fighting in Damascus is some of the worst since July, when rebels took over several neighborhoods in the capital in a striking attack. Within days, a regime counterattack pushed the rebels out of Damascus and recaptured the areas. Shortly after those battles, rebels moved on Syria's largest city, Aleppo, and it has become a major front in the civil war since then.
The attacks on Syria's two main cities since summer have demonstrated new organization and capabilities of rebel forces as well as a determination to press their uprising despite the deaths of more than 36,000 people in fighting over the last 19 months.
Rami Abdul-Rahman, who heads the Britain-based activist group Syrian Observatory for Human Rights, said the fighting in Damascus was concentrated in the southern neighborhood of Tadamon and the outskirts of the Palestinian refugee camp of Yarmouk.
Damascus-based activist Abu Qais al-Shami told The Associated Press via Skype that the fighting began Sunday night and went on continuously into Monday.
The Observatory and al-Shami said regime forces are backed by the Popular Front for the Liberation of Palestine-General Command (PFLP-GC) led by Ahmed Jibril, a radical Palestinian leader and staunch supporter of President Bashar Assad.
"Tadamon is being struck with shells, rockets and heavy machinegun fire," al-Shami said. "People are fleeing the area toward safer areas inside the Yarmouk camp."
The Observatory had no immediate word on casualties from Monday's fighting but said eight people were killed in Yarmouk Sunday night when several mortar rounds landed in the camp.
When Syria's unrest began last year, the country's half-million Palestinians at first struggled to stay on the sidelines. But in recent months, many Palestinians started supporting the uprising.
The PFLP-GC, however, has remained loyal to Assad's regime.
The popular committees in Yarmouk Camp, which are led by the PFLP-GC and represent camp residents and Palestinian factions, said the camp was attacked by "terrorist gangs" who claim to include anti-government Palestinians.
"The mercenaries who claim to have Palestinians among them" tried to infiltrate the camp Sunday but were repulsed by the popular committees, the statement said. When the rebel attack failed, they fired mortars that killed and wounded several people, it added.
"Those who are shelling the camp are terrorists" who want to displace the Palestinians again, PFLP-GC spokesman Anwar Raja told The AP in Damascus.
Syrian authorities blame the anti-government uprising that began in March last year on a foreign conspiracy. They accuse Gulf countries Saudi Arabia and Qatar, along with the U.S, other Western countries and Turkey, of offering funding and training to the rebels, whom they describe as "terrorists."
In northern Syria, An opposition figure also said there were clashes between rival rebel groups for control of the Bab al-Salameh border crossing point with Turkey that has been in the hands of rebels since July. The opposition figure spoke on condition of anonymity because of the sensitivity of the matter.
The opposition figure said Sunday's fighting was between the Northern Storm Brigade and the Amr bin al-Aas brigade, which has a large number of Muslim hardliners.
There are dozens of opposition groups and rebel brigades fighting in Syria's civil war. Rivalries are common though violent clashes are unusual.
A Turkish government official based in the border town of Kilis confirmed two Syrian rebel groups were "engaged in a power struggle," fighting each other for the control of the Bab el-Salameh border crossing. The official, who spoke on condition of anonymity, in line with government rules, said however, that Turkish officials were still trying to determine who the two groups were.
In the central province of Hama, a suicide attacker detonated his car in the village of Ziyara causing some deaths, state-run news agency SANA said. It added that the blasts occurred outside a state-run development agency. The Britain-based activist group Syrian Observatory for Human Rights said the suicide attacker targeted an army checkpoint and killed "tens of troops."
Elsewhere in Syria, activists reported air raids, shelling and clashes in the northern provinces of Aleppo and Idlib near the Turkish border to Deir el-Zour region in the east and Homs in the center.
(Copyright 2012 by The Associated Press. All Rights Reserved.)