There were few details on the attack, which was reported by the Local Coordination Committees activist group and the Britain-based Syrian Observatory for Human Rights.
The Observatory said it was aware of up to 100 killed from sources on the ground, but the group had only confirmed the names of 30 people so far.
Death tolls are nearly impossible to independently verify in Syria, where the government restricts journalists and where more than a year of violence has convulsed much of the country.
There were few details of the violence in Hama's Tremseh area.
Activists say more than 17,000 people have been killed since the uprising against President Bashar Assad began in March 2011, and he is coming under growing international pressure to stop the violence. But as the bloodshed continues, and the conflict morphs into an armed insurgency, hopes for a peaceful transition are dimming.
The latest report of violence came in the wake of the highest-level defector yet from President Bashar Assad's regime - his ambassador to Iraq.
Defections from the Syrian regime have stirred hopes in the West Assad's inner circle will start abandoning him in greater numbers, hastening his downfall.
But the tightly protected regime has largely held together over the course of the 16-month-old uprising, driven by a mixture of fear and loyalty.
The latest official to flee, Ambassador Nawaf Fares, announced that he was joining the revolution, asserting Thursday that only force will drive Assad from power.
"There is no road map ever with Bashar Assad, because any plan, any statement that is agreed on internationally he delays on and ignores," Fares told the Al-Jazeera satellite channel. "There is no way that he can be pushed from power without force, and the Syrian people realize this."
Syria's Foreign Ministry denounced Fares, saying he should face "legal and disciplinary accountability."
In Washington, State Department spokesman Patrick Ventrell hailed what he called the "first major diplomatic defection," adding: "We think this a wider sign that the regime is feeling the pressure. The pressure is up and the regime is really starting to fall apart."
Fares is the second prominent Syrian to break with the regime in less than a week. Brig. Gen. Manaf Tlass, an Assad confidant and son of a former defense minister, defected last week, but has not spoken publicly.
French Foreign Minister Laurent Fabius said Tlass has been in contact with the Syrian opposition. He would not comment on reports that Tlass was in Paris.
"I know that there is some closeness between the opposition and the general... Contact has been made," Fabius told journalists in Paris.
Assad's regime has suffered a steady stream of low-level army defectors, who have joined a group of dissidents known as the Free Syrian Army, now numbering in the tens of thousands. There have been several high-level defections in the past - including a Syrian fighter pilot who flew his plane to neighboring Jordan during a training mission in June in a brazen move.
Although the defections are notable, Assad's regime has remained remarkably airtight, particularly compared with the hemorrhaging of Moammar Gadhafi's inner circle in Libya in 2011.
Within weeks of the Libyan revolt, a number of Libyan ambassadors and other high-ranking officials quit the government, and many joined the opposition leadership. The early defection of huge sections of the army in eastern Libya gave the rebel movement a safe zone where they could freely organize their political and military strategies.
Syria has seen nothing similar. Part of the reason is the loyalty of the armed forces.
Unlike the armies of Tunisia and Egypt, Syria's military has stood fiercely by the country's leader as Assad faces down an extraordinary protest movement.
Assad, and his father who ruled before him, stacked key military posts with members of their minority Alawite sect over the past 40 years, ensuring the loyalty of the armed forces by melding the fate of the army and the regime.
The army has a clear interest in protecting the regime because they fear revenge attacks and persecution should the country's Sunni majority gain the upper hand.
But besides the military's loyalty, another factor that constrains a flood of defections is fear. Open dissent is dangerous in Syria, a country that crushed any rumblings of defiance even before the popular revolt started to threaten the Assad family's 40-year dynasty. The security forces, which are the backbone of the regime and drive the culture of fear and paranoia, will protect the leadership at all costs.
Defectors fear not only for their own lives, but for those of any family members left behind. When deputy oil minister Abdo Husameddine defected in March, he said in a video statement that he fully expected government forces to "burn my home" and "persecute my family."
Already, the conflict is believed to have killed more than 17,000 people since the crisis began in March 2011, according to activists' estimates. Although the revolt began with protests, it has morphed into an armed insurgency with scores of rebel groups across the country clashing with government troops and attacking their bases and convoys.
On Thursday, Syrian forces shelled the suburbs of the capital, Damascus, to flush the rebels out from areas where they have established a foothold. Troops pounded Mezzeh and Kafr Souseh in eastern Damascus with mortars, sending residents streaming out, activists said. They also targeted the Liwan, Qadam and Daraya neighborhoods from a nearby military airport.
Explosions could be heard though much of the capital and amateur videos posted online showed huge clouds of smoke rising from the targeted areas. The Britain-based Syrian Observatory for Human Rights reported three dead in the area, among more than 45 people killed across Syria on Thursday. At least 11 were government soldiers, it said.
The reports and videos could not be independently verified.
Also Thursday, Human Rights Watch said it had found evidence the Syrian government had fired cluster bombs in an area near the central city of Hama. The New York-based group said the munitions are clearly identifiable in amateur videos posted online, and that local activists said the area has been under government bombardment for weeks.
Cluster bombs explode in the air and drop dozens of "bomblets" over a large area, but these often do not explode on impact. They remain explosive, increasing the threat of later injury to civilians.
As the conflict grinds on, U.N. officials are growing more pessimistic over prospects for a diplomatic solution to the crisis, even though Assad's main backers, Russia and China, have signed on to the idea of a transition to democracy in Syria.
Despite incremental progress, a senior U.N. official said the U.N. Security Council is deeply divided on Syria policy, with Western diplomats still uncertain whether Moscow is any closer to cutting its ties with the Syrian government or using its considerable leverage with Damascus to end the conflict on terms unfavorable to Assad.
With diplomacy near a standstill, the U.N. observer mission in Syria is serving as little more than a bridge between the United Nations and the Assad government, the official said, speaking on condition of anonymity because he was not authorized to discuss diplomatic maneuvering with media.
Associated Press writers Ben Hubbard in Beirut, Matthew Lee in Washington and John Heilprin in Geneva contributed to this report.