(Adds Lebanese TV says Nabak captured, Islamists execute diesel vendor)
BEIRUT, Dec 9 (Reuters) - Syrian President Bashar al-Assad's forces have taken control of a highway connecting Damascus to the coast that is needed to extract hundreds of tons of toxic chemicals for destruction, a monitoring group said on Monday.
Fighting in Syria poses a hurdle to implementing an agreement between Damascus and the Organisation for the Prohibition of Chemical Weapons (OPCW) to remove the deadliest chemicals weapons by the end of the year to be destroyed.
The army started an offensive in mid-November to secure the highway, which passes through the mountainous area of Qalamoun, roughly 50 km (30 miles) north of Damascus, stretches along the Lebanese border and hosts many military bases and outposts.
The army has retaken the highway towns of Qara and Deir Attiyah from mostly Sunni Muslim rebels fighting to oust Assad, and has made inroads in and around the town of Nabak close to the road.
"The road is open but not safe," said Rami Abdulrahman, head of the anti-Assad Syrian Observatory for Human Rights monitoring group.
It remained vulnerable to rebel attack, he said.
Manar television, run by Lebanon's Shi'ite Hezbollah group and which has reporters embedded with the Syrian army, cited Syrian security sources on Monday saying that Nabak was now fully under army control and the highway was secure.
Rebels in the area did not respond to calls for comment. The government has never acknowledged that it did not fully control the road, a north-south artery that also links Damascus with the coastal stronghold of Assad's minority Alawite sect.
Syria has agreed under a deal arranged by the United States and Russia to dismantle its chemical arsenal and destroy all its 1,300 tons of sarin, mustard gas and other lethal agents.
The size of the stockpile, including 800 tons of industrial chemicals destined for incineration at commercial toxic waste plants, means it can only be transported by land and sea - using roads linking Damascus to the Mediterranean port of Latakia.
The Hague-based OPCW, which won the Nobel Peace Prize in October, has been charged with supervising the elimination of Syria's chemical arsenal. The U.S.-Russian agreement averted planned U.S. missile strikes after a sarin gas attack killed hundreds of people in the Damascus area in August.
The United States is donating a naval ship and equipment to destroy Syria's chemical arsenal, but there is no agreement yet about where it will anchor while the work is carried out.
Syria's conflict began with peaceful anti-Assad protests in March 2011 and developed into an armed insurrection when these were violently suppressed. More than 100,000 people have been killed and fighting has broadly settled into a stalemate.
Hardline Islamist groups have exploited the vacuum of power in many rebel-held areas and marginalised more moderate opposition groups.
The Observatory reported on Monday that members of the al Qaeda-linked Islamic State of Iraq and the Levant executed a diesel vendor in Idlib province after accusing him of blasphemy.
It said the man suffered from a mental illness and jokingly asked the militants if they thought he was "the God of fuel" after they complained that his supplies were watered down.
The Observatory sent a picture it said was of the man, with bullet holes to his head and on the street, where it said the militants left him. He had three children, it said. (Reporting by Oliver Holmes,; Editing by Alistair Lyon and Angus MacSwan)