UN observers in Syria were attacked on Tuesday with stones, metal rods and gunfire that blocked them from a besieged rebel-held town, where civilians were feared trapped by government shelling.

The Associated Press reported that the UN observers were not hurt as they were turned back by the assault on their vehicles by an angry crowd near the town of Haffa. The source of the gunfire was not clear.

Activists blamed regime loyalists for the attack. The violence raised questions about the ability of about 300 unarmed monitors to provide a useful assessment of a country that is spiraling toward civil war.

“All UN observers are now back at their bases and are secure,” Sausan Ghosheh, a spokeswoman for UN observers in Syria, was quoted by AP as having said. She added monitors have been trying to reach Haffa since June 7.

Meanwhile, the UN’s peacekeeping chief, Herve Ladsous, said the Syrian conflict had escalated into civil war.

“Yes, I think one can say that,” Ladsous was quoted as having told news agencies in an interview.

A spokesman for Ladsous, Kieran Dwyer, told AP that UN observers have seen a steep rise in violence and a dangerous shift in tactics by both sides in Syria in the last five days.

He said the Syrian government, intent on wresting back control of rebel-held areas, is shelling heavily populated districts and using attack helicopters over cities “with devastating impact on civilians.” The opposition, in turn, is increasingly coordinating attacks against government forces and civilian infrastructure, and “the conflict has reached all parts of Syria virtually,” he said.

The deteriorating situation in Haffa has raised alarm in the past eight days, and Washington said on Monday that regime forces may be preparing a massacre in the village, which is about 20 miles from President Bashar Assad’s hometown of Kardaha in Latakia.

Activists said government forces were firing mortar rounds into the village.

The UN’s special envoy to Syria, Kofi Annan, renewed calls Tuesday for the bloodshed to end. Annan brokered a peace plan that was supposed to go into effect April 12 but never took hold.

“It is totally unacceptable and it must stop,” Annan’s spokesman, Ahmad Fawzi, was quoted by AP as having said, “and that is why Annan has invited governments with influence to raise the bar to another level, to the highest level possible, and twist arms if necessary, to get the parties to implement the plan.”

Meanwhile on Tuesday, the U.S. accused Russia of escalating the Syrian conflict by sending attack helicopters to President Bashar Assad’s regime.

“We are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria, which will escalate the conflict quite dramatically,” Secretary of State Hillary Clinton said in Washington.

There was no immediate reaction from the Russian Foreign Ministry. Moscow insists that any arms it supplies to its Damascus ally are not being used against anti-government protesters in the 15-month-old uprising.

On Monday, the UN charged that Assad’s armed forces kill and sexually attack children and use them as human shields.

“In almost all recorded cases, children were among the victims of military operations by government forces, including the Syrian armed forces, the intelligence forces and the shabiha militia, in their ongoing conflict with the opposition, including the Free Syrian Army,” the UN stated in its annual report on children and armed conflict.

Syrian opposition sources claimed last week that aircraft belonging to the Syrian Air Force dropped toxic material into the province of Daraa, which smells like sulfur and causes drowsiness and unconsciousness.

It was also reported that Assad’s forces had used unidentified gas shells on civilians in Daraa, Hama and Deir ez-Zor.

In a meeting with the Druze community in the Golan Heights on Saturday, Deputy Minister MK Ayoub Kara was presented with evidence that clearly indicates the use of chemical weapons by the Assad regime, including photos of big black clouds suspected of containing chemical material that were fired over the areas where battles between rebels and regime forces have been taking place.