American troops stationed along Iraq's border with Syria (search) are coming under increasing mortar attack from shells fired from Syrian territory, but it's unclear who's responsible, U.S. officers said Thursday.

The 82 mm mortar rounds have been fired at U.S. and Iraqi positions in and around Husaybah (search) in the far west of Iraq's Anbar province, said Lt. Col. Chris Woodbridge, commander of the 1st Battalion, 7th Marine Regiment.

"Who exactly is firing these mortars, we do not know. But what we do know is that the point of origin of these rounds is on the Syrian side of the border," said Woodbridge, 39, of Brooklyn.

There has been no evidence linking the Syrian military to the attacks, he said. However, the Syrian military has the capability to determine who is launching the mortars and act against them, Woodbridge said.

"Syrian authorities should be the ones to go after them, no question about it," he said.

The mortar attacks come at a time of increased U.S. pressure on Syria to stem cross-border infiltration and movement of militants into Iraq.

The number of cross-border mortar attacks has increased in recent weeks with the latest coming Tuesday night, Woodbridge said. In one night alone, five mortar rounds landed near U.S. troops in Husaybah, he said.

The mortar rounds have largely missed their target and there have been no U.S. casualties, U.S. officers said.

A high-level U.S. delegation visited Syria last month to ask the government to institute tighter controls on its side of the border and to stop militants and their money from entering Iraq.

Following those talks, U.S. Secretary of Defense Donald H. Rumsfeld (search) called the Syrians "unhelpful" and accused them of "facilitating terrorists moving back and forth, money moving back and forth" to Iraq. Rumsfeld made the remarks during a speech at Council on Foreign Relations in New York.

While Syria has posted hundreds of extra troops along the frontier, its officials have said it is impossible to seal off such a large area.

Woodbridge maintains that the official Syrian presence at the border is scarce, consisting of a few patrols at the border crossing.

The U.S. Marines, who patrol a 250-mile stretch of mostly desert terrain between the two countries, say the Syrians have not been helpful in securing their side of the border.

"They should be more active in patrolling their side of the border, in searching vehicles, in detaining suspicious characters," Woodbridge said.