Syrian rebels have battled Shias in a village in the country's east, killing over 60 people including civilians.
The fighting highlights the increasingly sectarian nature of the country's civil war.
Activists say the dead were mostly pro-government militiamen, without specifying whether the non combatants had been killed deliberately or were caught in the crossfire.
But a Syrian government official denounced the attack on the Shia-section of Sunni-majority Hatla village as a "massacre" of civilians.
A Syrian government helicopter fired at least two missiles at a border town in Lebanon.
The Britain-based Syrian Observatory for Human Rights said at least 60 people, mostly Shia fighters but also ordinary villagers, were killed in Hatla in the oil-rich province of Deir el-Zour that borders Iraq.
Most of the armed rebels in Syria are from the country's Sunni majority.
President Bashar Assad has retained core support among the minorities, including his own Alawite sect, an offshoot of Shiite Islam, along with Christians and Shias.
Amateur videos released by activists showed rebels standing in front of burning homes captioned, "Setting fire to the houses of Shias." The video shows at least two bodies, one of them of a bearded man.
The videos appeared genuine and corresponded to other Associated Press reporting on the events depicted.
Thousands of rebels took part in the attack and at least 10 of them were killed in the fighting, said the Observatory.
An activist based in Deir el-Zour said the rebel attack was in retaliation for an attack on Monday by Shias from Hatla that killed four rebels.
In Damascus, a government official said the rebels "carried out a massacre against villagers in which older people and children were killed."
The fighting in Deir el-Zour came a week after Syrian troops, backed by Lebanon's militant Shia Hezbollah group, captured the strategic town of Qusair.
Qusair is located near the Lebanese border after nearly three weeks of fierce battles that killed dozens of troops, rebels and Hezbollah members.
Hezbollah's involvement in the Qusair battle underlined the group's commitment in support of President Assad's regime.
It edged the civil war in Syria closer to a regional sectarian conflict pitting the Middle East's Iranian-backed Shia axis against Sunnis.
Building on its victory in Qusair, the Syrian military has shifted its attention to try to clear rebel-held areas in the province of Homs.
Homs is considered a linchpin area linking Damascus with regime strongholds on the Mediterranean coast, and the northern city of Aleppo.
The Observatory reported heavy clashes in the center of Hom city, mostly in the neighbourhood of Wadi Sayeh.
The fighting appeared to be an attempt by government forces to separate two main rebel-held areas in the city, Khaldiyeh and the city center.
The state-run news agency SANA said troops killed several gunmen in the town of Talbiseh north of Homs.
The Observatory reported fighting and shelling in the northern Damascus neighborhood of Barzeh, which has witnessed clashes between troops and rebels over the past weeks.
It said there were casualties without giving figures.
Meanwhile, a Syrian helicopter violated Lebanese airspace and fired two missiles toward the central square in the village of Arsal, wounding one person, Lebanese military officials said.
Residents said three missiles were fired.
The Arsal attack is the latest incident in which Lebanon has been pulled into the war next door. Scores of rebels and civilians who fled from Qusair have taken refuge there.
The town is predominantly Sunni Muslim, and support for the Syrian rebels runs high.
Elsewhere, The United States is debating what more it might do to help the Syrian opposition in its civil war against the government.
He said: "We are determined to do everything that we can in order to help the opposition to be able ... to save Syria,"
Mr Kerry said: "People are talking about what further options might be exercised here ... but we don't have anything to announce at this moment."