The Syrian town of Haffeh was smoldering and nearly deserted on Thursday after days of clashes between government forces and rebels, while activists reported more army assaults on pro-opposition areas across the country.

United Nations monitors had been trying to enter the town after several days of fighting but were forced to turn back on Tuesday when a crowd attacked them.

They finally gained access on Thursday to find state buildings burnt down, shops abandoned and a body lying in the street. Smoke rose from destroyed buildings and burnt-out cars littered the roads. There were signs of heavy bombardment.

Only a handful of residents could be seen and one man said 26,000 people had fled.

Rebels pulled out of the town this week saying the thousands of remaining citizens risked being killed in cold blood, a warning echoed by the United States.

Violence has surged in recent weeks after government forces and allied militia launched offensives to regain territories controlled by the opposition and rebels abandoned a ceasefire negotiated by international envoy Kofi Annan in his efforts to ease the conflict between President Bashar al-Assad and the movement fighting to end his family's four decade rule.

The British-based Syrian Observatory for Human Rights, which has a network of activists across the country, said 44 people were killed on Thursday.

Most were civilians and rebels but there were also three security personnel among the dead, it added.

Assad's forces have used troops, tanks and helicopters to hit rebel-controlled suburbs near the capital Damascus, parts of eastern Deir al-Zor province and villages in the northern and western parts of Aleppo province, near the Turkish border.

In Douma, about 15 km (10 miles) from Damascus, activists said tanks had entered the city outskirts and government forces were battling rebel fighters. At least two people were killed and 15 wounded, they added.

"It is a war today," said an activist who called himself Ziad, speaking on Skype over the thump of shelling and the rattle of machinegun fire. "There are 10 tanks on the outskirts, but the rebels have destroyed one of them."

The uprising against Assad's rule began as a peaceful pro-democracy movement in March 2011 but in the face of a crackdown by his forces has turned into an armed insurgency.

"There has been a dangerous escalation of armed violence across Syria," said Sausan Ghosheh, spokeswoman for the U.N. monitors observing the now-collapsed ceasefire.

"They (residents) want the violence to stop and so do we, but the U.N. Supervision Mission cannot impose a ceasefire. The path of non-violence is a choice for the welfare of all Syrians."

The United Nations says more than 10,000 people have been killed by government forces, while Syria says at least 2,600 members of the military and security forces have been killed by what it calls foreign-backed "Islamist terrorists".

State news channel Syria TV said security forces had arrested a man who was part of Jabhat al-Nusra, a little known militant group that has claimed several suicide bombings in Syria. It said the man was planning to blow himself up at a mosque on Friday.

World powers are divided over the next move.

Russia and China, both permanent members of the U.N. Security Council with veto power, have blocked efforts by Western powers to condemn Assad or call for his removal.

Diplomats said world powers were working towards holding a crisis meeting on Syria in Geneva on June 30 to try to get the Annan plan back on track.

IRAN THE STICKING POINT

Annan, who represents the United Nations and the Arab League, has called for a Contact Group to be convened as soon as possible, but the United States is opposed to the involvement of Iran, Syria's main ally in the region.

Two diplomats told Reuters they were hoping to have a meeting on the 30th, but a third said Iran's participation was still a sticking point.

Annan's spokesman Ahmad Fawzi said the envoy was having urgent meetings to reach consensus on the shape and formula for the meeting. If one was held, it would aim to 'give teeth' to the Annan plan, not to create a new one, he said.

U.S. Secretary of State Hillary Clinton said on Tuesday Washington had information Russia was supplying Syria with helicopters, which have been used in government assaults on towns and cities.

Syria's ambassador to Moscow told Reuters on Thursday that Russia was 'not delivering any helicopters to Syria'.