DAMASCUS - Syrian Kurds urged the opposition on Saturday to halt a siege against them by Islamist rebels, as the UN condemned the killing of dozens of children across the country over the past week.

The Kurdish National Council, a pro-opposition umbrella group of Syrian Kurdish parties, condemned what it said was an ongoing assault "against unarmed civilians" by jihadist insurgents on the northern town of Ras al-Ain.

It said the rebels, who came across the border from Turkey, were shelling the town indiscriminately, and called on the main opposition National Coalition and the Free Syrian Army to "pressure these militants to stop this criminal war which is detrimental to the Syrian revolution."

On Saturday, one rebel was killed in shelling and fierce clashes that pitted the jihadist Al-Nusra Front against Kurdish fighters, the Syrian Observatory for Human Rights said.

A resident of Ras al-Ain told AFP both sides appeared to be preparing for a fierce battle, with Kurdish fighters building fortifications and digging trenches, and rebels bringing in reinforcements from across the border.

Turkey, which supports the revolt against Syrian President Bashar al-Assad, is home to a sizeable Kurdish minority whose demands for greater independence it has moved to suppress, notably in air strikes on Kurdish militants groups.

Activists say Turkey may be using jihadists in Syria to fight its own battle against the Kurds.

Elsewhere, Syrian warplanes raided the eastern outskirts of Damascus as army reinforcements arrived in Daraya, a key battleground near the capital, the Observatory said.

The violence came a day after 149 people were killed, according to the Observatory, including 103 civilians and 18 children.

The United Nations Children's Fund condemned the killing of children in Syria..

"A series of reports from Syria this week underlines the terrible price children are paying" in a conflict that has ravaged the country for 22 months and killed more than 60,000 people, UNICEF said.

"Media reports (Friday) from the scene of mass killings in the village of Hasawiya outside Homs said whole families were among the dead," it added.

"UNICEF condemns these latest incidents in the strongest terms, and once again calls on all parties to ensure civilians -- and children especially -- are spared the effects of the conflict."

The Observatory, which relies on a network of activists and medics for its information, says 3,538 children have been killed since the start of the revolt in March 2011.

Meanwhile, Syria's foreign ministry criticised a petition by UN rights chief Navi Pillay and 58 countries calling for a war crimes case against Damascus to be opened at the International Criminal Court.

"The Syrian government regrets the persistence of these countries in following the wrong approach and refusing to recognise the duty of the Syrian state to protect its people from terrorism imposed from abroad," it said.

Those countries, are "hindering the Syrian national dialogue for a peaceful settlement of the crisis as proposed by Assad on January 6." Those talks would only include opposition forces tolerated by the regime, disqualifying the vast majority of the country's rebels.

Rebels offered a stark reply on Saturday by assassinating Khaled al-Hilal, a local official responsible for arranging any such talks in the southern province of Daraa.