* Syrian army surrounded UNESCO World Heritage site
* Residents say battle over gas pipeline, sectarian feud
HOMS PROVINCE, Syria, Jan 29 (Reuters) - President Bashar al-Assad's forces have surrounded rebels near the already war-damaged Crusader castle of Crac des Chevaliers, a UNESCO World Heritage site in central Syria, residents said on Wednesday.
Crac des Chevaliers suffered mortar hits last year when rebels from the town of al-Hosn below the hill-top castle hid behind its thick stone walls, built for battles hundreds of years ago. The violence had died down until this week.
Syria's nearly three-year-old conflict has devastated whole city neighbourhoods and many ancient sites, including Aleppo's medieval covered market and its Umayyad mosque. Looting has threatened tombs in the desert town of Palmyra and Roman temples have been damaged.
On Tuesday, rocket, artillery and tank fire battered the town of al-Zara, not far from al-Hosn, a resident of al-Zara said, asking not to be named. Warplanes later bombed the area between al-Zara and al-Hosn, targeting rebels there, he said.
Although fighting has inched closer to Crac des Chevaliers, the castle has not been hit during the two-day-old offensive by troops and local pro-Assad militiamen besieging al-Hosn.
A fighter from the National Defence Forces, an Assad loyalist paramilitary group, said the offensive aimed to secure a gas pipeline which runs through al-Zara and which was attacked by rebels in December to disrupt supplies.
But others in the area linked the fighting to a dispute between villages in Homs, a religiously-mixed province where the increasingly sectarian conflict between Assad's Alawite minority and mostly Sunni Muslim rebels has increased tensions.
Al-Zara and al-Hosn are mostly Sunni, but the surrounding villages form part of Syria's 10 percent Christian minority. Christians are generally wary of the rise of Islamist militants among the rebels, although some have fought against Assad.
This month, masked gunmen from al-Hosn decapitated a young man from the nearby Christian village of Marmarita, residents said. A few days later, pro-government militiamen from Marmarita killed a man from al-Hosn and dragged his body behind a car through Christian hamlets.
Two weeks of Sunni-Christian reprisals then led to this week's army-backed offensive in the area, residents said.
The war has killed at least 130,000 people, driven more than a third of Syria's 22 million people from their homes and made half dependent on aid, including hundreds of thousands trapped by fighting. (Reporting by a journalist in Syria whose name has been withheld for security reasons; Writing by Oliver Holmes)
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