The U.S. accused Russia of escalating the Syrian conflict by sending attack helicopters to President Bashar Assad's regime, and U.N. observers were attacked Tuesday with stones, metal rods and gunfire that blocked them from a besieged, rebel-held town where civilians were feared trapped by government shelling.

There was no immediate reaction from the Russian Foreign Ministry on the accusation by U.S. Secretary of State Hillary Rodham Clinton. 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.

"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," Clinton said in Washington.

The U.N. 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 U.N. said. 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 U.N. observers are now back at their bases and are secure," said Sausan Ghosheh, a spokeswoman for U.N. observers in Syria. She said monitors have been trying to reach Haffa since June 7.

During the conflict, the observers have been prevented from entering other flash point areas. Last week, it took days for the monitors to reach Mazraat al-Qubair, where nearly 80 people were reported slain, because government troops and residents blocked them.

As the conflict deteriorates into a murderous grind, regional power brokers from Iran to Turkey risk getting drawn into the fight. Diplomatic hopes have rested on Russia _ Syria's most important ally and protector _ agreeing on a transition plan that would end the Assad family dynasty, which has ruled Syria for more than four decades.

But Moscow has rejected outside forces to end the conflict or any plan to force regime change in Damascus. Despite withering criticism from the West, it insists that any arms it supplies to Syria are not being used against civilians.

The U.N.'s special envoy on Syria, Kofi Annan, asked governments with influence to "twist arms" to end the bloodshed. But there are few signs the diplomatic pressure is having any measurable effect.

"The longer this violence continues, the more dangerous it becomes not only for the country and the Syrian people but the region," Annan's spokesman, Ahmad Fawzi, told reporters in Geneva. "It's dangerous and the red light is flashing."

The deteriorating situation in Haffa has raised alarm in the past eight days, and Washington said Monday that regime forces may be preparing a massacre in the village, which is about 20 miles (30 kilometers) from Assad's hometown of Kardaha in Latakia province along the Mediterranean coast.

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

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.

Violence has escalated significantly in recent days as the government fights to reassert control of pockets of resistance across the country. Syrian forces fired mortars at protesters Tuesday in the eastern city of Deir el-Zour, killing at least 10 people, activists said. Clashes also were reported in central Homs province.

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 their bodies. Other videos showed some of the wounded being treated at a hospital.

The state news agency SANA blamed the Haffa violence on terrorists _ the term it uses to describe rebels _ who had attacked residents in the village. The report also said a reporter and a cameraman for the pro-government Ikhbariya TV were wounded when bullets hit their car in Haffa on Monday.

Throughout the uprising, violence has rattled Latakia, an area of profound importance to the regime. Latakia province is the heartland of the Alawite minority to which Assad and the ruling elite belong, although there is a mix of religious groups.

Syria's Sunni majority makes up the backbone of the opposition, and minorities such as Alawites and Christians have generally stuck to the sidelines, in part out of fears that they will be marginalized _ or even face retribution _ if Sunnis take over.

As the violence worsens, world powers have struggled to find ways to end the bloodshed. Annan brokered a peace plan that was supposed to go into effect April 12 but never took hold. More than 13,000 people have died in Assad's crackdown on the uprising that began in March 2011 as part of the Arab Spring, according to activist groups.

Annan renewed calls Tuesday for the bloodshed to end.

"It is totally unacceptable and it must stop," spokesman Fawzi 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."

He didn't specify the countries that might still have leverage with the Assad regime, but Russia, China and Iran are Syria's closest and strongest allies.

Fawzi said it was up to the government to take the first step to end the violence.

"The stronger party should send a strong signal in good faith and stop the violence, and the stronger party in this case is clearly the government of Syria," he said.

Syria has so far appeared largely impervious to the chorus of condemnation that began shortly after government forces launched a crackdown on peaceful protests. Economic sanctions have had little effect.

Russia and China have used their veto power at the U.N. Security Council to block strong action against Damascus to prevent any international military intervention against Syria.

But the U.S. and its allies also have shown little appetite for getting involved in another Arab nation in turmoil.

British Foreign Secretary William Hague said Britain and other Western powers were not seeking a foreign military intervention, despite his warning Monday that all options must remain open if diplomacy doesn't work.

"Clearly, we are not looking for any foreign military intervention, and we should not think about it in terms of another Libya," Hague said during a two-day visit to Pakistan.

"All our efforts are going into supporting a peaceful transition in Syria and a peaceful solution, because any violent solution would clearly involve many more deaths and a great deal more hardship for the Syrian people," he said.

Besides the potential for more violence, there also is a real concern it could spill over to other countries in the region.

The International Organization for Migration said more than 5,000 Syrian refugees have now crossed into northern Iraq, and their numbers are growing.

"With the help of international agencies we have managed to provide them with basic necessities, including shelter, food and water since the camp opened at the beginning of April," Shakir Yasseen Yasseen, director general of the Regional Government's Bureau of Migration and Development. "But our prognosis is that they will not be going back home anytime soon."

___

AP writers Bassem Mroue in Beirut, Frank Jordans in Geneva, Bradley Klapper in Washington and David Stringer in London contributed to this report.