Syrian troops have killed 50 "terrorists" in an ambush in the largely rebel-controlled countryside east of Damascus, according to state media.

"An army unit killed 50 terrorists in an ambush while they tried to flee Mediya village," SANA news agency reported late Wednesday, using the standard official term for the rebels.

The Syrian Observatory for Human Rights said 30 people, all of them men ,had been killed in the operation, backed by fighters of Lebanon's Hezbollah.

But it said it could not confirm whether all of the dead were rebels.

A Hezbollah source confirmed that the group's fighters had taken part in the operation and said 30 rebels had been killed.

The Shiite militant group has deployed thousands of fighters to Syria to support President Bashar al-Assad's regime against the mainly Sunni rebels.

Countryside east of Damascus known as Eastern Ghouta has remained largely in rebel hands, despite repeated efforts by the army to oust them.

In August 2013, the area was hit by a string of chemical weapons attacks that killed hundreds of people.

The attacks sparked US threats of military action that were defused only by the government's agreement to dismantle its chemical arsenal.

US 'horrified' by Syrian regime raids, slams Assad
Washington (AFP) Nov 27, 2014 - The United States said Wednesday it was "horrified" by Syrian regime air strikes in the stronghold Raqa that killed at least 95 people, slamming the government for its "continued slaughter."

The bombing Tuesday was the deadliest by President Bashar al-Assad's air force since Islamic State jihadists seized the city last year and declared it their capital.

It was not clear how many Islamic States members were killed, but the Syrian Observatory for Human Rights said more than half of the dead were civilians.

The US State Department condemned the strikes, and said the regime had no value for human life.

"We are horrified by the reports that the Assad regime's airstrikes yesterday in Raqa, Syria killed dozens of civilians and demolished residential areas," State Department spokeswoman Jen Psaki said in a statement.

"The Assad regime's continued slaughter of Syrian civilians further exposes its callous disregard for human life."

She said Assad had lost his right to rule, and said his actions should be held accountable.

"We've been clear that Assad long ago lost all legitimacy to govern and that the Syrian regime must be held responsible for its brutality and atrocities against the Syrian people."

Psaki blamed Assad's government for continued human rights abuses and violating international law, accusing the regime of "murder, hostage-taking, enforced disappearances, torture, rape, sexual violence and the indiscriminate use of barrel bombs."

The United States has built up a coalition of more than 60 countries in the fight against the Islamic States militants who have seized swathes of territory in Iraq and Syria.

US President Barack Obama has rejected an alliance with Assad against the Islamic State group, arguing that any such pact would backfire.

Obama has said communication with the Assad regime was limited to informing them when they use Syrian air space in operations against the Islamic State jihadists.