?For almost three weeks, Syria's central city of Homs has been pounded by shelling from the forces of President Bashar al-Assad, leaving hundreds dead, according to opposition activists. Judging from a video clip posted online, one weapons analyst says Assad's forces are using the biggest mortars in the world.

The video was first flagged in the Christian Science Monitor, which was told by a Human Rights Watch official that the regime forces are using the Russian-made 240mm "Tulip." In the clip, two men are standing in rubble holding up the fanned tails of the exploded ordnance.

Peter Falstead of Jane's Defence Weekly says the tail fins look "very much like the tail fins from SM-240," also known as the "Tulip Tree" developed by the Soviets in the 1970s. Today it is the largest mortar system used by any military in the world, and the Syrian army is believed to have up to 10 in service.

"If you wanted to strike at rebel-held positions in a built-up area to which you had no line of sight, and you had no regard whatsoever for the killing of innocent civilians, then I guess the SM-240 would be a weapon of choice," Falstead told ABC News.

Few of the self-propelled SM-240s (also known as the M-1975) remain in service, Jane's says, due to its short range and slow firing (around one shell per minute). All-told, the system weighs 60,000 pounds,  its range is between 2,600 and 5,900 feet, and it can fire shells weighing between 300 and 500 pounds.

By comparison, Falstead says the largest mortars used by the U.S. Army are 120mm, noting that they do have howitzers of a larger caliber.