BEIRUT - Angry crowds blocked UN observers from reaching an embattled rebel-held town in Syria on Tuesday, hurling stones and metal rods at the monitors’ vehicles. The vehicles came under fire as they drove away from Haffa, but the source of the gunfire was not clear, the UN said.

None of the observers were injured.

The situation in Haffa has raised alarm over the past eight days, and there are concerns civilians are stuck in the area while the regime and rebel fighters battle for control. Washington said Monday that regime forces may be preparing a massacre in rebel-held Haffa - a village about 20 miles from Assad’s hometown of Kardaha.

It’s not clear why the crowd wanted to prevent the observers from entering, but the Britain-based Syrian Observatory for Human Rights said earlier that residents of a nearby village were trying to block the observers.

Citing a network of sources on the ground, the Observatory said the residents were mostly regime loyalists.

Calls to the area did not go through Tuesday. The government restricts journalists from moving freely, making it nearly impossible to independently verify accounts from either side.

Also Tuesday, Syrian forces pelted the eastern city of Deir el-Zour with mortars as antigovernment protesters were dispersing before dawn, killing at least 10 people, activists said.

The offensives were part of an escalation of violence in recent weeks that has brought more international pressure on President Bashar Assad’s regime over its brutal tactics against the opposition. In a new report, the UN accused the government of using children as human shields. It said children have been victims of detention, torture, and sexual violence.

Amateur video of the mortar attack on Deir el-Zour showed some of the dead in a street as survivors screamed in panic and tried to remove the bodies. Other videos showed some of the wounded receiving treatment at a hospital. The Local Coordination Committees activist group and the Syrian Observatory for Human Rights said 10 people died in the shelling.

Both sides of the 15-month-old revolt to oust Assad have ignored an internationally brokered cease-fire that was supposed to go into effect April 12 but never took hold. The United States and its allies also have shown little appetite for getting involved in another Arab nation in turmoil.

Syria is veering ever closer to an all-out civil war as the conflict turns increasingly militarized. Already more than 13,000 have died since March 2011, according to activist groups.

The UN special envoy for Syria, Kofi Annan, has asked governments with influence to “twist arms’’ to halt the escalating violence in the country, his spokesman said. Annan is also working to convene a diplomatic meeting soon to discuss the situation in Syria amid worsening fighting between government troops and opposition forces. Earlier this year, he put forward a six-point peace plan, including the cease-fire, to try to end the violence.

“It is totally unacceptable and it must stop, 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,’’ his spokesman told reporters in Geneva.