Fighters from the military wing of Hamas are reportedly training and fighting alongside Syrian rebels, marking a total about-face by the Islamist group, which was once one of Syrian President Bashar Assad’s staunchest allies.

Citing anonymous diplomatic sources, The Times of London reported Friday that members of Hamas’s Izz ad-Din al-Qassam Brigades paramilitary wing are taking part in Syria’s two-year civil war alongside opposition fighters.

“The Qassam Brigades have been training units very close to Damascus. These are specialists. They are really good,” the paper quoted a high-level Western diplomat close to the conflict as saying.

Some sources told the paper that Hamas advisers were using their tunnel-building skills honed in Gaza, where weapons and goods are brought into the Strip via underground conduits, to prepare the ground for a rebel assault on central Damascus.

Another source said Hamas fighters were actively battling alongside rebels in Palestinian refugee camps in Damascus and Aleppo.

Hamas, whose leadership mostly decamped from Damascus at the beginning of the conflict, denied the claim to the paper, saying there were no Hamas fighters anywhere in Syria. The group said a Hamas fighter killed in Idlib had left the movement before joining up with the rebels.

In June, top Hamas operative Kamal Hussein Ranaja was killed on the outskirts of Damascus, though the group blamed Israel for the death.

As the civil war in Syria has deepened, most of the country’s half-million Palestinians have backed the rebels, while some groups — such as the Popular Front for the Liberation of Palestine-General Command — have been fighting on the government side.

Hamas politburo chief Khaled Mashaal has never publicly taken sides, but in early 2012 slipped out of Syria for Qatar, drawing an angry response from Damascus.

A number of Arab sources told The Times that Hamas may be getting involved in Syria at the behest of Qatar’s leaders, who have overwhelmingly backed the rebels.

In February 2012, Hamas’s Gaza leader Ismail Haniyeh openly called to support the rebels, aligning himself alongside other Sunni groups that have struck out against the Alawite Assad and his Shiite backers.

“I salute all the nations of the Arab Spring and I salute the heroic people of Syria who are striving for freedom, democracy and reform,” he said in Cairo at the time.