BEIRUT (Reuters) - Two suicide bombers from al Qaeda-linked groups blew themselves up at an army post in the Syrian town of al-Nabak on Wednesday, halting an advance by President Bashar al-Assad's forces near the Syrian-Lebanese border, activists said.
The battles raging around the Qalamoun range spanning Syria and Lebanon are part of a much-anticipated offensive by Assad's forces to secure a highway linking Damascus to the province of Homs and thus consolidate their grip on central Syria.
The Syrian Observatory for Human Rights said suicide bombers attacked the army checkpoint in explosive-laden cars, killing and wounding several soldiers, but gave no precise toll.
The British-based monitoring group said the blasts coincided with a rebel attack on the nearby government-held town of Deir Attiya, which had not previously been involved in the fighting.
"This seems to have temporarily stopped the army's advance," said the Observatory's head, Rami Abdelrahman, of the checkpoint assault.
Militants with the hardline Nusra Front said their group, along with another al Qaeda-linked organization, the Islamic State in Iraq and the Levant, had sent hundreds of fighters to the Qalamoun front in recent weeks.
"There could be more special attacks like what you saw today," said a source linked to the Nusra Front, who declined to be named. "The regime has a better chance of winning but the fighters have prepared some surprises with the help of God."
Sources on both sides of Syria's 2-1/2-year-old conflict say the Qalamoun battle will be a longer struggle than Assad's swift capture of the border town of Qusair this year, when Lebanese Shi'ite Hezbollah guerrillas fought beside Syrian troops.
But the fighting has already affected neighboring Lebanon - more than 6,000 Syrians have fled there to escape shelling.
A twin suicide bombing hit the Iranian embassy in Beirut on Tuesday, killing at least 23 people and wounding 146.
The al Qaeda-linked Abdullah Azzam Brigades, a Sunni Muslim group based in Lebanon, claimed the attack and said it was to pressure Iran and its Shi'ite proxies to pull out of Syria.
Shi'ite Iran has given Assad money and military support in his fight against the mostly Sunni rebels. Tehran also funds Hezbollah, which is involved in several battles in Syria.
So far Hezbollah fighters - seen as critical to the Qusair victory - have not come out in force in Qalamoun.
An opposition activist in the area said this may be because the Syrian army offensive is focused on the highway. Assad's forces seized the town of al-Qara on Tuesday and may only need the town of al-Nabak to cement control of the road.
"It's in their interest to take the few towns they need and let the rebels move into the mountains," said the activist, who only identified himself Ibn al-Qalamoun (son of Qalamoun).
"They know the rebels are their own worst enemies right now - they'll fight each other for the remaining smuggling routes into Lebanon and be distracted with that," he said.
Rebels are on the back foot across much of Syria, and critics in the opposition say they have themselves to blame. They point to a surge in infighting in northern Syria, where rebels once held large swathes of territory.
Assad is also pushing an offensive around Aleppo. Earlier this month the rebels lost several strategic border towns in the northeast to Kurdish militias seeking an autonomous region.
(Reporting by Erika Solomon; Editing by Alistair Lyon)