Just 10 families live in the village in a cluster of about 20 homes, Lt. Khaled Ali, a spokesman for the rebel Free Syrian Army's military council in Hama, told CNN .
The Local Coordination Committees said that 35 of the dead are from one family.
An activist who was less than two miles from Mazraat al-Qabeer also said that at least 78 people had been killed the village. He said he was working with others to evacuate the bodies, many of which were burned.
He said 38 of the victims were men, and the rest were women and children.
In a rare call for direct military action, the opposition Syrian National Council said the rebel Free Syrian Army should "escalate battlefield action" to ease pressure on civilians "under siege [by] shelling and assaults in the provinces of Hama, Latakia and Homs."
Echoing descriptions of the massacre of 108 civilians at Houla on May 25, which U.N. observers attributed to Assad's troops and loyalist 'shabbiha' militia, the Syrian Observatory for Human Rights said of the Qabeer killings: "Shabbiha headed into the area after the shelling and killed dozens of citizens, among them women and children."
Ali said Qabeer is surrounded by Alawite villages that support the regime and that the militia members came from two of the neighboring villages.
Soon after the opposition groups reported the slaughter, Syrian state TV reported that government forces were headed to the village to help.
"An official source in Hama says that the security forces have responded to calls for help and complaints from the people in al-Qabeer ... and raided a terrorist cell and killed a number of them and confiscated their weapons," state TV reported.
At Houla, near Homs, nearly half the victims were children.
In that earlier case, Assad himself condemned the atrocity but denied any hand in it and blamed opponents, whom he describes as foreign-backed "terrorists."
Killings of civilians from the Sunni Muslim majority, who largely support the uprising, by shabbiha drawn mostly from Assad's minority Alawite sect, who identify with the Shi'ites of Iran, have raised fears of an Iraq-style sectarian bloodbath and reinforced a wider regional confrontation between Iran and the mainly Sunni-led Arab states of the Middle East.
The failure of a ceasefire brokered by U.N. envoy Kofi Annan in March to halt the bloodshed has raised questions over its continued worth and Annan, the former U.N. secretary-general, is to brief the Security Council Thursday in New York.
A 300-strong force of U.N. truce observers has been in Syria for weeks and can be expected to investigate the accounts from Mazraat al-Qabeer, which came in after nightfall in Syria.
There was no immediate comment from the government, and events on the ground are difficult to verify as Syria tightly restricts access to international media.
Activists, including the Observatory based in Britain, called for an immediate investigation: "The Syrian Observatory for Human Rights calls on the international monitors to go immediately to the area. They should not wait to tomorrow to investigate this new massacre," it said in a statement.
"They should not give the excuse that their mission is only to observe the ceasefire, because many massacres have been committed during their presence in Syria."
U.N. diplomats said they expected Annan to present the Security Council with a new proposal to rescue his failing peace plan by creating a "contact group" of world and regional powers.
Some rebel groups, which have helped escalate what began as popular demonstrations for democracy into what is approaching a civil war, have lost faith in any ceasefire calls and are calling for more foreign arms and other support.
Western leaders, wary of new military engagements in the Muslim world and especially of the explosively complex ethnic and religious mix that Syria represents, have offered sympathy but show no appetite for taking on Assad's redoubtable armed forces, which can call on Iran and Russia for supplies.
Separately, ministers and envoys from 15 countries and the European Union agreed at a meeting hosted by Turkey in Istanbul on Wednesday to convene a "coordination group" to provide support to the opposition but left unclear what it may involve.
U.S. Secretary of State Hillary Clinton said a transition strategy in Syria must include Assad's full transfer of power, a senior U.S. State Department official said.
Speaking to reporters after the meeting, the official said Clinton had also told the group that transition in Syria must include a fully representative interim government that would lead to free and fair elections.
France also announced at the meeting it would hold a full "Friends of Syria" meeting in Paris on July 6, the official said.
Clinton was among officials from Europe, Turkey and Arab states who discussed "additional steps" including coordination on an "effective and credible transition process" to lead to a "democratic, post-Assad Syria", a Turkish statement said, adding that the group would be represented at a meeting in Istanbul next week of Syrian rebels.
Annan hopes his new idea can prevent a total collapse of his plan for a truce and negotiated political solution, U.N. diplomats said. The core of the proposal, diplomats said, would be the establishment of a contact group that would bring together Russia, China, the United States, Britain, France and key regional players with influence on Syria's government or the opposition, such as Saudi Arabia, Qatar, Turkey and Iran.
By creating such a contact group, envoys said, Annan would also be trying to break the deadlock among the five permanent council members that has pitted veto powers Russia and China against the United States, Britain and France and prevented any meaningful U.N. action on the Syrian conflict.
It would attempt to map out a "political transition" for Syria that would lead to Assad stepping aside and the holding of free elections, envoys said. One envoy said the idea was "vaguely similar" to a political transition deal for Yemen that led to the president's ouster.
The main point of Annan's proposal, they said, is to get Russia to commit to the idea of a Syrian political transition, which remains the thrust of Annan's six-point peace plan that both the Syrian government and opposition said they accepted earlier this year, but have failed to implement.
While Russia has repeatedly said it is not protecting Assad, it has given no indications that it is ready to abandon him. Assad has proven to be a staunch Russian ally and remains a top purchaser of weapons from Russian firms, and diplomats say Moscow continues to reward him for his loyalty.
"The thought is one that we've had for a little while, which is that you need to bind Russia into some sort of transition strategy on Syria," a senior Western diplomat said.
An unnamed diplomat leaked further details of Annan's proposal to Washington Post columnist David Ignatius, who said that if the contact group agreed on a transition deal for Syria, it would mean "Assad would presumably depart for Russia, which is said to have offered him exile".
It was not immediately clear if the idea of Russian exile for Assad was something Annan was pushing or if it was Ignatius's speculation. The Post article said that another option for Assad would be to seek exile in Iran, Damascus' other staunch ally.
To contact the editor, e-mail: