Troops loyal to Syrian President Bashar al-Assad made new advances Sunday on the rebel stronghold town of Qusayr near the border with Lebanon, reports said.
Activists say the army, backed by pro-regime militia and the Lebanese Shiite movement Hezbollah, is aiming to crush insurgents in villages surrounding Qusayr, and to then launch an assault on the town itself.
Three new shells launched from the Qusayr area landed in eastern Lebanon, the official National News Agency reported, in what rebels described as retaliation for Hezbollah's involvement in Syria's war and for allegedly firing from across the Lebanese border.
Two of the shells hit the outskirts of the town of Hermel and the other hit the Qasr area, said the Lebanese agency.
Qusayr has been under rebel control for more than a year. It is strategic because it lays on a route linking Damascus to the coast.
"Loyalist troops backed by Hezbollah have taken control of important villages near Qusayr," said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
Qusayr-based activist Hadi al-Abdallah told AFP that Hezbollah fighters were instrumental to the army's advance.
"The only reason why the regime is advancing in the Qusayr area is because of Hezbollah's troops. Hezbollah fighters advance on the ground, while the (Syrian) air force gives them cover," Abdallah said via the Internet.
Among the flashpoint villages were Radwaniyeh, seized on Saturday, Burhaniyeh and Tal al-Nabi Mando, which is strategic because it lies on a hilltop and overlooks rebel-held areas.
The army meanwhile secured the road linking the Lebanese-Syrian border along the Orontes river to the west of Qusayr.
The fighting has left dozens of people dead this week.
In a statement, the main opposition National Coalition urged Hezbollah to "immediately withdraw its forces from Syrian territory", adding that the Shiite movement's involvement in Syria's conflict "could drag Lebanon and the region into an open-ended conflict with disastrous consequences".
"The Coalition calls on the Lebanese government... to take all the necessary measures to put a stop to the actions of Hezbollah, which is flagrantly involved (in the conflict) on the side of the Assad regime," the statement added.
The situation in the border area could "explode", said the group, while calling on rebel "Free Syrian Army brigades in Homs province to show restraint and to respect Lebanon's sovereign borders".
There has been no comments from Hezbollah about the latest fighting but in the past it has denied any involvement in the Syria war.
Syria's pro-regime Al-Watan newspaper insisted, meanwhile that the army controls villages around Qusayr. "There is a big change in the army's tactics. It has become more precise in securing its objectives," it said.
Al-Watan said the rebels, who the regime refers to as "terrorists", suffered a setback nearby.
"More than 100 of 500 terrorists who tried to storm the Dabaa airport were killed in an ambush set by the army, which is now chasing down the terrorists who fled," it said.
The Observatory earlier this week reported rebels seized control of the Dabaa military airport, which has fallen out of use.