* Syrian army advances along Lebanese border
* Shells land in Lebanon (Adds civilians killed, army advances)
TRIPOLI, Lebanon, March 20 (Reuters) - Dozens of wounded Syrian rebels crossed over a river into Lebanon on Thursday after government soldiers ambushed them as they tried to flee a besieged area, Lebanese medical sources said.
The ambush was part of a campaign by the Syrian army and allied militias to secure towns and villages along the Lebanese border as well as a highway from the capital Damascus to the coast that is vulnerable to rebel attack.
The rebels were fleeing the area of al-Hosn in Homs province, which the Syrian army has surrounded, the medical sources said. Syrian state television said soldiers had killed 11 rebels to escape al-Hosn.
The sources said that 41 wounded rebels were taken into a hospital in north Lebanon. A further eight were carried in dead or succumbed to their wounds after managing to escape Syria into Wadi Khaled, said the medical sources - a hospital employee and a medic who requested anonymity.
As the fighters fled, Syrian army shells hit villages in Wadi Khaled, Lebanon's National News Agency said.
The Syrian army campaign has accelerated a deterioration in Lebanon's security and stability, raising local sectarian tensions as thousands of refugees flood into the country.
Lebanese frontier towns have been used by rebels to recuperate but have also been attacked by Syrian helicopters and rocket attacks.
Lebanese Shi'ite militant group Hezbollah has sent fighters into Syria to support President Bashar al-Assad, while Syrian rebels and their Sunni Lebanese allies have set off bombs in Shi'ite areas and fired rockets at Shi'ite towns inside Lebanon.
Al-Hosn sits just below the war-damaged Crusader castle of Crac des Chevaliers, a UNESCO World Heritage site. The castle suffered mortar hits last year when rebels hid behind its thick stone walls, built for battles hundreds of years ago.
The Syrian Observatory for Human Rights, an anti-Assad monitoring group based in Britain, said that displaced civilians were also killed on Thursday while trying to escape al-Hosn but it did not have a confirmed death toll.
More than 140,000 people have been killed in the three-year-long Syrian conflict, which has become increasingly sectarian as rival regional powers have backed either Assad, a member of the Shi'ite offshoot Alawite sect, or the majority Sunni rebels who oppose him. (Reporting by Nazih Saddiq; Writing by Oliver Holmes,; Editing by Angus MacSwan)