Syrian security forces are maintaining a heavy presence in the northwestern Idlib province after an offensive on army defectors killed about 250 people, including a large number of civilians, activists say.
The Syrian National Council, an umbrella group representing opponents of President Bashar al-Assad's government, on Wednesday called for "immediate action" by the Arab League and the UN Security Council to condemn and halt what it called "horrific massacres" conducted by Syrian forces in Idlib and other areas.
The United States said it was "deeply disturbed" by the ongoing violence, which it said was a violation of Syria's implementation of the league's pleace plan.
Al Jazeera's Zeina Khodr, reporting from the Turkish city of Antakya, along the border with Syria, said the Syrian military's operations had been focused on Idlib because it was attempting to regain control over what had become a stronghold for army defectors.
"It is clear that army defectors have taken control over some towns and villages, almost as though they have created some sort of safe area, where protesters from other regions were seeking a safe haven and where defectors were able to operate from," she said.
She also noted that Idlib was a "strategic area" because it was located near Turkey, which has become a serious critic of the Syrian government and a supporter of the opposition movement.
"If defectors really take over the province, it would give them a supply line to Turkey and create a safe zone which they have been demanding for some time now," our correspondent said.
Bodies 'burnt and beheaded'
Reports of violence are difficult to verify because communications to the region have mostly been shut down. Many people in the province also lack electricity and water, according to activists.
Alaa El Din Al Youssef, a Syrian opposition member in Idlib, described the government's attack on the area of Jabal al-Zawiyah on Tuesday as a "massacre".
"Civilians were surrounded by security forces who killed 100 of us. The corpses of those killed were left in the streets and the mosques and we are not allowed to bury any of them."
"Some of those killed cannot be recognised. Some were burnt and some beheaded with their hands tied. We are really scared because the area might be stormed once again."
Activists said the army offensive had targeted the village of Kfar Owaid, about 50km from the border with Turkey. It is part of the rugged mountainous region of Jabal al-Zawiyah, which has been the scene of clashes between troops and army defectors and intense anti-government protests for weeks.
Troops began attacking the region on Saturday, residents said.
The army said it had completed its military operation in Jabal al-Zawiyah after tens of people were killed in clashes with "armed men".
Al Jazeera cannot independently verify reports from Syria because of restrictions on reporting inside the country.
The Local Co-ordination Committees activist network said 15 people were killed in different Syrian provinces across the country on Wednesday - five in Hama, four in Homs, three in Idlib and three in Deraa.
The latest fighting comes as the Arab League prepares to send a team of observers to Syria on Thursday as part of an effort to halt the violence.
Syria agreed to the move on Monday after the Arab group threatened to submit the issue to the UN Security Council.
Ahmed bin Helli, the league's deputy secretary-general, said he hoped the mission would enable a resolution to the crisis to be found without any need for international intervention.
In-depth coverage of escalating violence across Syria
But Khodr said the SNC's latest statement suggested they had lost patience with the Arab League initiative.
"The SNC now thinks the Arab League should work on ways to topple the government, and not work on peace plans because for them that is no longer on the table," she said.
France joined the US and other nations in denouncing what it said was an "unprecedented massacre", and urged Russia to accelerate talks for a UN Security Council resolution on the crisis.
French foreign ministry spokesman Bernard Valero said: "Everything must be put in motion to end this murderous spiral into which Bashar al-Assad is dragging his people, deeper each day."
Russia has proposed a Security Council resolution that would denounce violence from both sides.
France has called this "unacceptable," seeking instead a resolution that would directly pin the blame for the violence on the authorities and threaten strong international sanctions on Damascus.