The United Nations on Tuesday accused Syrian troops of using children as "human shields," as UN chief Ban Ki-moon demanded access to the town of Al-Heffa amid fears of a new "massacre" by regime forces.

Branding Damascus as one of the worst offenders on its annual "list of shame" of conflict countries, the UN said in a report that Syrian children as young as nine had been victims of killing and maiming, arbitrary arrest, detention, torture and ill-treatment, including sexual violence.

"Rarely, have I seen such brutality against children as in Syria, where girls and boys are detained, tortured, executed, and used as human shields," Radhika Coomaraswamy, UN special representative for children in armed conflict, told AFP before releasing the report.

Ban said in a statement meanwhile that "intensive military operations" by government forces against the central province of Homs and firing from helicopters on other towns had caused heavy civilian casualties.

Residents and activists say government helicopter gunships have strafed rebel positions in Al-Heffa, a town of 30,000 near the border with Turkey, and tanks were parked on the outskirts.

The UN Supervision Mission in Syria (UNSMIS) said it had received reports of "a large number of civilians, including women and children trapped inside the town and are trying to mediate their evacuation."

One Syrian activist broke down in tears as she told AFP via Skype that tanks were parked on the edge of Al-Heffa.

"They have never come this close before," Sem Nassar said, adding: "There's only one doctor working to treat the wounded in the town," and that most residents had fled.

Ban joined UN-Arab League envoy Kofi Annan in demanding that unarmed military observers from UNSMIS be let into Al-Heffa.

Such reports prompted Washington to voice concerns Assad's regime is planning to carry out new atrocities, after the massacre of 55 people last week in Al-Kubeir and at least 108 near Houla on May 25-26.

"The United States joins joint special envoy Kofi Annan in expressing deep alarm by reports from inside Syria that the regime may be organising another massacre," US State Department spokeswoman Victoria Nuland said.

Nuland said the tactics showed the Assad regime, cracking down on the most severe threat to his family's four-decade rule, was "increasingly desperate."

Human rights groups estimate that about 1,200 children have died during the 15-month uprising against President Bashar al-Assad, whose brutal crackdown on Arab Spring-inspired protests has been widely condemned.

The UN report said that Syrian government forces rounded up dozens of boys aged eight to 13 before an attack on the village of Ain al-Arouz in Idlib province on March 9.

The children were "used by soldiers and militia members as human shields, placing them in front of the windows of buses carrying military personnel into the raid on the village," it said.

Quoting witnesses, the UN report said Syrian military and intelligence forces, as well as pro-government shabiha militiamen, surrounded the village for an attack that lasted more than four days.

Among the 11 dead on the first day were three boys aged 15 to 17. Another 34 people, including two boys aged 14 and 16 and a nine-year-old girl, were detained.

"Eventually, the village was reportedly left burned and four out of the 34 detainees were shot and burned, including the two boys aged 14 and 16 years," the Children in Armed Conflict report said.

The report was completed before the Houla massacre on May 25, when 49 of the 108 victims were said to be children, some as young as two and three, who were shot in the head or had their skulls smashed with blunt instruments.

On the ground on Tuesday, 10 civilians were killed when the Al-Jbaible neighbourhood of the eastern city of Deir Ezzor was hit by mortars fired by regime forces, the Syrian Observatory for Human Rights said.

The mortar attacks come a day after 12 people were killed, including three children, when a car bomb exploded in Al-Joura neighbourhood of Deir Ezzor, the Britain-based group said.

It said that Hreitan in the northern province of Aleppo was subjected to violent shelling on Tuesday morning, adding that clashes with rebels at the entrances to the town have led to "heavy losses among regime troops."

Syrian army helicopters fired on rebel stronghold towns on Monday, leaving at least 106 people dead nationwide, including 77 civilians and 23 Syrian troops, activists said.

Reports of high daily death tolls are becoming the norm in Syria where over 14,000 people have been killed since the anti-regime revolt erupted in March 2011, according to the Observatory.