Syria is now in a full-scale civil war, UN peacekeeping chief Herve Ladsous said on Tuesday, as UN observers reported they were fired on as they tried to enter a town feared to be the target of a new massacre.

The news came as the Syrian government accused Washington of encouraging more massacres in the strife-torn country, which Damascus always attributes to "armed terrorists," and of meddling in its internal affairs.

Asked whether he believed Syria is in a civil war, Ladsous told a small group of reporters: "Yes I think we can say that. Clearly what is happening is that the government of Syria lost some large chunks of territory, several cities to the opposition, and wants to retake control."

"There is a massive increase in the level of violence," Ladsous said.

On the ground, the UN Supervision Mission in Syria said observers trying to reach the northwestern town of Al-Haffe were driven back by an angry crowd of people who threw rocks and metal bars at them, and were then fired on by unknown assailants.

"As they were leaving the area, three vehicles heading towards (northwest) Idlib were fired upon," the UNSMIS statement said. "The source of fire is still unclear."

Earlier, the Syrian Observatory for Human Rights said "residents of the pro-regime village of As-Sheer blocked the road and prevented the UN observer team from reaching Al-Haffe," also speaking of the observers being targeted with stones.

The Observatory's Rami Abdel Rahman said people "lay down on the road blocking access to the vehicles of the observers," who then began looking for another route into to the town, in the province of Latakia.

It was not immediately clear if UNSMIS and the Observatory were talking about the same incident.

For its part, the Syrian foreign ministry said the "government reaffirms its adherence" to UN-Arab envoy Kofi Annan's peace plan and "will not allow armed groups and their supporters to attack or threaten the UN observers or prevent them from completing their task."

Free Syrian Army rebels have clashed with regime troops at the town's edges in fierce fighting, local activist Abu Mohammed told AFP via Skype, adding that "the town is completely besieged but regime forces have so far been unable to enter it."

At least 120 people have been killed in the fighting in Al-Haffe over the past week, including 68 troops, 29 civilians and 23 rebel fighters, with hundreds wounded, according to Abdel Rahman.

The Observatory also reported troops using heavy artillery and helicopter gunships in their attacks on Al-Haffe, where activists feared a massacre would be committed if regime troops managed to enter the restive town.

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

The UN observer mission 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 Annan in demanding that unarmed military observers from UNSMIS be allowed into Al-Haffe.

The United States has voiced concerns that the 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 in late May.

"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," State Department spokeswoman Victoria Nuland said.

The French foreign ministry spokesman echoed the US warnings.

"We share concerns about the preparation of new massacres," Bernard Valero said, adding that "we more than ever support the Annan plan."

In its statement, the Syrian foreign ministry struck back by saying "the US administration is pushing forth with its flagrant interference in Syria's internal affairs and its backing of armed terrorist groups.

"US statements distort the truth and what is happening on the ground while encouraging armed terrorist groups to carry out more massacres ... not only in Al-Haffe but throughout the country," a statement added.

Meanwhile, the UN 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.

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

"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.

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

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.

At least 36 people were killed in shelling and clashes across Syria on Tuesday, 24 of them civilians and 12 soldiers, the Observatory said.

More than 14,100 people have been killed in Syria since the anti-regime revolt erupted in March 2011, according to the Observatory.

[A YouTube image grab shows smoke billowing from the flashpoint Syrian city of Homs which activists said was being attacked by regime forces. AFP Photo]