* Release part of deal to free jailed women -watchdog
* Many Christians have tried to stay on war's sidelines
* Islamist rebels had claimed to have taken the nuns (Adds delay to nuns' expected arrival at border)
By Alexander Dziadosz
BEIRUT, March 9 (Reuters) - An operation to release about a dozen nuns held by rebels in Syria for more than three months began on Sunday, security sources and church officials said, but hit an unexplained delay.
A Lebanese security source had said the nuns had been taken to the Lebanese town of Arsal earlier in the week and would head to Damascus on Sunday accompanied by the head of a Lebanese security agency and a Qatari intelligence official.
By late Sunday, however, they had not arrived at the Syrian border. The reason for the hold-up was not immediately clear.
The nuns went missing in December after Islamist fighters took the ancient quarter of the Christian town of Maaloula north of Damascus.
After being held in the Greek Orthodox monastery of Mar Thecla in Maaloula, they were reportedly moved to the rebel-held town of Yabroud, about 20 km (13 miles) to the north, which is now the focus of a government military operation
Speaking to reporters at the border, Syrian Greek Orthodox Bishop Louka al-Khoury welcomed the reported release of the nuns. "What the Syrian army achieved in Yabroud facilitated this process," he said.
Shortly after the nuns disappeared, Islamist rebels said they had taken them as their "guests" and that they would release them soon.
The British-based Syrian Observatory for Human Rights monitoring group identified the rebels who took the nuns as militants from the Nusra Front, al Qaeda's affiliate in Syria.
The Observatory and a rebel source in the area said the release of the nuns had been agreed as part of a swap in which the government would free scores of women prisoners.
"The deal is for the release of 138 women from Assad's prisons," the rebel source said, referring to Syrian President Bashar al-Assad.
In December, the nuns appeared in a video obtained by Al Jazeera television, saying they were in good health, but it was not clear under what conditions the video had been filmed.
Syrian state television devoted significant coverage to the expected release on Sunday, but made no mention of any prisoner exchange agreement. It broadcast live footage from the Lebanese border and interviews with church officials, including one who denounced the West as only believing "in the dollar".
A montage of Christian imagery including churches, a statue of the Virgin Mary and murals of Jesus was set against music and described Syria as a "cradle of the monotheistic faiths."
Syria's Christian minority has broadly tried to stay on the sidelines of the three-year-old-conflict, which has killed over 140,000 people and which has become increasingly sectarian.
But the rise of hardline Islamists among the overwhelmingly Sunni Muslim opposition has alarmed many. Assad, whose minority Alawite sect is an offshoot of Shi'ite Islam, has portrayed himself as a bulwark against militant and intolerant ideologies. (Reporting by Alexander Dziadosz, Kinda Makieh and Mariam Karouny; Editing by Alistair Lyon)