The Lebanese Shia group Hezbollah has sent its fighters on an offensive along Lebanon's eastern border with Syria, fighting alongside government forces to dislodge opposition fighters from strongholds in the mountainous area.
Battles erupted between Hezbollah fighters and Syrian troops on one side and the opposition fighters on another in the strategic town of Qalamoun on Sunday.
Qalamoun is a key supply route for both Hezbollah and the Syrian government.
It is also important for the government because it wants to secure a key northwestern road which connects the capital Damascus to Homs, and then on to President Bashar al-Assad's stronghold of Latakia on the western coast.
Al Jazeera's Omar al-Saleh, reporting from the Bekaa Valley in Lebanon, said the battle could be a long one and could easily spill over into Lebanon and deepen the political infighting and sectarian tensions there.
"The mountain range of Qalamoun is vast and rugged. The Syrian rebels are adopting guerrilla warfare tactics. They use the mountainous terrain as their hideout," he said.
"That could prove hard for the Syrian government jets and Hezbollah fighters to fully control or clear the area. Despite Hezbollah's attacks, Syrian rebels remain powerful in the area."
A team of Associated Press news agency journalists travelling with Hezbollah into Syria found fighters proudly showing newly dismantled booby traps and food quickly left behind by Syrian fighters as Hezbollah commanders promised further advances they say protect Lebanon.
Some 3,000 fighters are in the Qalamoun region, almost equally split between the Nusra Front and the Islamic State of Iraq and the Levant (ISIL) group, a Hezbollah commander recently told AP in Beirut.
He said Hezbollah and Syrian troops have Qalamoun surrounded from the north, the east and the south, as well as part of the west, squeezing the Syrian fighters who remain.
Deadly clashes in Idlib
In another Syria-related development, opposition fighters stormed a hospital complex in northwestern Idlib province on Sunday where about 250 government loyalists have been trapped for two weeks now, the Syrian Observatory for Human Rights said.
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The UK-based activists' network said the fighters entered the facility in Jisr al-Shughour, the rest of which they captured almost two weeks ago.
Al Jazeera cannot independently verify the reports.
Jisr al-Shughour is west of the Idlib city, the provincial capital of Idlib, which fell to fighters in late March.
The Syrian Observatory said the fighters carried out the raid on the southwestern edge of Jisr al-Shughour where they are engaged in heavy fighting with government forces holed up inside the complex.
Syrian State TV said government forces had closed all roads leading to the town, which was captured by the opposition fighters late last month.
It is the first time that the fighters have managed to penetrate the complex.
According to the monitoring group, 33 air strikes were launched around the flashpoint areas on Sunday morning.
Source: Al Jazeera and agencies