Thirteen members of the same family, mostly women and children, have been killed by pro-government forces in north-west Syria, activist groups say.
Some reports say some of the victims were burnt alive, in the incident in Bayda near the coastal city of Banias.
The family members were said to be from a mostly Sunni village, but in an area where government supporters have been accused of trying to clear out Sunnis.
The new violence came as the UK said moderate rebels deserved support.
Even though the government of President Bashar al-Assad may have got “stronger” in recent months, there was a “stalemate” on the ground, Prime Minister David Cameron told the BBC.
Mr Cameron said work must continue internationally to try to find a solution.
UK military chiefs have warned of the risks of arming rebel groups.
Mr Cameron said there was “too much extremism” among the opposition, but moderate groups still deserved support.
In other developments:
* Syrian government forces shell the town of Ariha in the north-western Idlib province, killing at least 18 people, with video shootage posted by activists appearing to show numerous mutilated bodies
* Syria’s state news agency Sana says government forces have killed “a large number” of rebels from the militant group Jabat al-Nusra near the western town of Adra. north-east of Damascus. Activists say 28 rebels and one Republican Guard officer died
* Kurdish forces in northern Raqqa province release the local commander of the al-Qaeda-linked Islamic State of Iraq and al-Sham, in return for 300 kidnapped Kurdish civilians
According to the UK-based Syrian Observatory for Human Rights – which relies on a network on opposition activists on the ground – at least three men from the Fattouh family in Bayda were shot dead by government forces and militia in the latest violence.
UK PM: “We need to do more to help those parts of the opposition that want a free, pluralistic Syria”
Women and children from the same family were then crowded into one room in a house, where they were all killed, it said.
Some reports said they were burnt alive when the house was set on fire, others, that they were shot beforehand.
The reported deaths came after clashes in Banias in which some loyalists died.
The BBC’s Jim Muir in neighbouring Lebanon says this suggests the attack involved an element of revenge.
The village of Bayda is described as a pocket of Sunni Muslims in the coastal province of Tartus, which is dominated by Mr Assad’s minority Alawite sect.
Both Bayda and Banias were the scene of what were described as sectarian massacres of Sunni families in May, in which more than 100 people died.
UN says more than 90,000 people have been killed in Syria since the uprising again President Assad began in 2011. A further 1.7 million have been forced to seek shelter in neighbouring countries.