DAMASCUS - Syrian regime forces shelled the northwestern town of Al-Haffe for an eighth day on Tuesday, sparking fears of an impending massacre and UN demands that its observers be granted access to the flashpoint.

Rami Abdel Rahman of the Syrian Observatory for Human Rights said that regime forces were using heavy artillery against the town while massing reinforcements in preparation for a ground assault.

Residents said that helicopter gunships were strafing rebel positions in Al-Haffe and said they feared a massacre if troops managed to enter the town, considered strategic because of its proximity to Qardaha, President Bashar al-Assad's home town.

Abdel Rahman said hundreds of rebel Free Syrian Army fighters are active in and around Al-Haffe, a town of about 30,000 people in Latakia province, setting the scene for a violent confrontation.

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."

UN chief Ban Ki-moon joined UN-Arab League peace envoy Kofi Annan in demanding that unarmed military observers from UNSMIS be allowed into Al-Haffe.

One Syrian activist broke down in tears as she said via Skype that army tanks were parked on the edge of Al-Haffe.

"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.

Washington voiced concerns that President Bashar al-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 in Washington.

The Britain-based Observatory said troops also pounded a neighbourhood of the eastern city of Deir Ezzor with mortar fire on Tuesday, killing 10 civilians including a young girl, while five other civilians were killed in attacks on the central city of Homs.

"The situation is horrific, murderous," an activist in Homs who identified himself as Abu Bilal said via Skype.

Abu Bilal said 400 civilians -- including women and children -- were trapped in a school in the Jourat al-Shiah neighbourhood.

"There are no fighters there, but still it's being shelled," he said.

"We're scared of a big massacre. We've never seen so much shelling before," he said.

The United Nations meanwhile Tuesday accused Syrian troops of using children as "human shields", as it branded Damascus one of the worst offenders on its annual "list of shame" of conflict countries.

Syrian children as young as nine had been victims of killing and maiming, arbitrary arrest, detention, torture and ill-treatment, including sexual violence, the UN said in a report.

"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, said before releasing the report.

Annan, who brokered a faltering six-point plan aimed at ending the bloodshed, wants to bring together world and regional powers to put pressure on Syria's leader, his spokesman said Tuesday.

"We hope that this contact group meeting will take place soon," Ahmad Fawzi told reporters in Geneva, adding that a date, venue and list of participants had not yet been decided but were "coming together."

Diplomats said Annan sees the five permanent members of the UN Security Council -- Britain, China, France, Russia and United States -- taking part along with Iran, Turkey, Saudi Arabia, Qatar and others.

Russian Foreign Minister Sergei Lavrov, whose country enjoys close commercial and military ties with Tehran, had earlier pressed for Iran to be included despite strong reservations from the United States, France and Britain.

Iran on Tuesday welcomed the Russian proposal.

"We support any method which helps positive talks between the government and the opposition in an appropriate environment, and also suitable reform actions for realising the people's ambitions," Iranian foreign ministry spokesman Ramin Mehmanparast told reporters.

Meanwhile, Syria's main opposition coalition called for "peaceful protests" across the world on Wednesday against Russia, for providing "political cover" to the Assad regime.

Reports of high daily death tolls are becoming the norm in Syria where over 14,100 people have been killed since the anti-regime revolt erupted in March 2011, according to the Observatory.