BEIRUT // Syrian air force jets fired four rockets at a remote section of the border with Lebanon yesterday, 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 strike near the Bekaa Valley town of Arsal, whose Sunni Muslim residents mainly support Syrian rebels trying to topple Bashar Al Assad.
A US official confirmed the attack, denouncing it as "a significant escalation" of the conflict.
"Regime jets and helicopters did fire rockets into northern Lebanon.... This constitutes a significant escalation in the violations of Lebanese sovereignty that the Syrian regime has been guilty of," US state department spokeswoman Victoria Nuland said.
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 the conflict.
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.
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".
Also yesterday, the US administration said that it would not hold back Britain, France or anyone else from arming Syria's rebels.
John Kerry, the US secretary of state, said: "The United States does not stand in the way of other countries that made a decision to provide arms."
Mr Kerry cited an imbalance in the fighting. Mr Al Assad's regime was attacking with tanks, scud missiles and aircraft – which the rebels don't have. He also said Iran, Hizbollah and Al Qaeda-related elements were helping the Assad regime.
Syrian regime forces yesterday resumed shelling parts of Damascus as security forces carried out arrests of students at Damascus University, a monitoring group said.
"There is shelling of the areas of Maadaniya and Jubar by regime forces," the Syrian Observatory for Human Rights said, reporting sniper fire and casualties in the city's northern Barzeh district too.
The Palestinian refugee camp of Yarmuk in the south of the city also came under rocket fire overnight.
Elsewhere in the capital, the Observatory said, regime forces carried out "a campaign of arrests of a number of students from the University City student housing in the Mazzeh district" of west Damascus.
The Observatory director, Rami Abdel Rahman, said "intelligence forces this morning entered University City", which is home to Damascus University students from different areas. The group also said that five people were killed and about 10 others wounded by "heavy shelling on areas in the city of Irbin",east of the capital. Regime forces have launched a broad campaign in the region around the capital in recent months, trying to unseat rebels from the bases they are using to attack Damascus.
In central Hama province, opposition fighters gained control of the Tel Hamamiyat checkpoint on the eastern outskirts of Karnaz.
The Britain-based Observatory, which relies on a network of activists and medical sources on the ground to compile its figures, said a total of 126 people had been killed across Syria on Sunday.
*With additional reports from the Associated Press and Agence France-Presse