Syrian air force jets fired four rockets at a remote section of the border with Lebanon on Monday, security sources said, four days after Damascus warned it may strike at Syrian rebels taking refuge across the frontier.

There were no immediate reports of casualties from the air strike near the Bekaa Valley town of Arsal, whose Sunni Muslim residents mainly support Syrian rebels fighting to topple Syrian President Bashar Assad.

Lebanon has a policy of "dissociation" from the two-year civil war in Syria but officials say they feel their country is increasingly at risk of being dragged into a conflict that the United Nations says has killed 70,000 Syrians.

Syria's Foreign Ministry said on Thursday that a "large number" of militants had crossed Lebanon's northern border into the Syrian town of Tel Kalakh last week.

"Syria expects the Lebanese side to prevent these armed terrorist groups from using the borders as a crossing point, because they target Syrian people and are violating Syrian sovereignty," it said in a message to the Lebanese government, according to state media.

It said Syria's "patience is not unlimited", even though "Syrian forces have so far exercised restraint from striking at armed gangs inside Lebanese territory."

'Significant escalation'

The U.S. State Department confirmed later Monday that Syrian government aircraft fired rockets into northern Lebanon and it described the incident as "a significant escalation."

"We can confirm what you are seeing in the press, that regime jets and helicopters did fire rockets into northern Lebanon," State Department spokeswoman Victoria Nuland told reporters at her daily briefing. "This constitutes a significant escalation in the violations of Lebanese sovereignty that the Syrian regime has been guilty of. These kinds of violations of sovereignty are absolutely unacceptable."