Syrian air force warplanes bombarded rebel-held targets close to a major chemical weapons facility on Friday in fighting that highlights the perils facing an international mission to eliminate President Bashar al-Assad's chemical arsenal.
The Organisation for the Prohibition of Chemical Weapons (OPCW), which was awarded the Nobel Peace Prize on Friday, is due to visit 20 sites across Syria to verify the destruction of 1,000 tonnes of chemical agents and precursors.
The mission in the midst of a civil war that has killed more than 100,000 people is an unprecedented challenge for the OPCW, whose members came under fire near Damascus in August.
The OPCW experts have visited three undisclosed sites in their first week of operation and say that Syrian authorities have been cooperating. But they will face great challenges reaching locations in rebel-held or disputed territory.
The air raids struck the town of Safira, on the edge of a sprawling military complex believed to hold chemical weapons production facilities, after overnight clashes between rebel fighters and Assad forces in a nearby village, activists said.
The army has fought hard to retain control of the Safira military complex and is now trying to recapture the town from rebel brigades including the al Qaeda-linked Nusra Front and Islamic State in Iraq and the Levant.
Unless they succeed in pushing those fighters back, any attempt by the OPCW experts to visit Safira would be risky.
"Right now it would be impossible with the clashes and the air strikes, especially as there is a strong presence of the Islamic State and the Nusra Front, who don't believe in anything called the international community," said Rami Abdulrahman of the Syrian Observatory for Human Rights.
OPCW head Ahmet Uzumcu appealed on Wednesday for temporary ceasefires to allow the mission to operate safely in conflict areas, but Abdulrahman said the Nusra Front and Islamic State fighters would not respect calls for a halt in hostilities.
A statement issued by the OPCW and United Nations on Friday said the mission has made "good progress" verifying information submitted by Syria and that material destroyed so far included munitions and chemical weapons production equipment.
A Western diplomat in the Middle East who is following the process said the cooperation shown by authorities would need to be reciprocated by rebels in the form of unhindered access to sites in conflict zones and complete safety for their work.
"There are clear signs from the joint team in Syria that the government is delivering on its responsibilities," he said.
"However divided the opposition might be it would look very bad if the government were seen to cooperate fully while inspections were held up because of problems with the opposition."
None of the actual chemical agents has yet been destroyed and the United States has suggested that the OPCW use a US-made mobile destruction unit to carry out the work, which is seen as preferable to shipping the chemical agents out of Syria because it is illegal for most countries to import them.
Syria agreed to eliminate its chemical stockpile after the United States threatened air strikes in response to sarin gas attacks which killed hundreds of people in Damascus six weeks ago.
Washington blamed Assad's forces for the incident, but Syrian authorities said rebels were behind it.
While the international spotlight has focused on the elimination of those weapons, fighting has raged on with conventional arms.
Syrian army troops and Shi'ite militia fighters loyal to Assad captured two southern suburbs of Damascus on Friday, killing at least 70 people, opposition activists
The capture of the two districts, located between two major roads heading south towards Jordan, strengthens Assad's hold on major supply lines and puts pressure on rebel brigades under siege for months in suburbs east and southwest of the city.
Buoyed by opposition divisions and the receding prospect of US military strikes, Assad has tried to tighten his grip on the country's centre, the Mediterranean coast and the capital - a major area of operations for his foreign Shi'ite allies.
In the suburb of Saqba, east of Damascus, activists said six people were killed by artillery or mortar fire which struck as they were leaving a mosque after the main weekly Muslim prayers.
Both sides of Syria's conflict have been accused of war crimes. Human Rights Watch said on Friday the killing of 190 civilians by rebels in Latakia province two months ago amounted to evidence of crimes against humanity.
HRW said many of the dead had been executed by militant groups, some linked to al Qaeda, who overran army positions at dawn on Aug. 4 and then moved into 10 villages nearby where members of Assad's Alawite sect lived.