Shells fired from Syria landed overnight in northern Lebanon after an exchange of fire along the border, a senior Lebanese security official told AFP early Tuesday.
Gunmen had opened fire on both sides of the border at several illegal crossing points, said the security official.
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The Syrian shells were fired into Lebanon following the cross-border gun battle, the source added.
Several witnesses told AFP they had seen dozens of families fleeing their homes in the frontier region of Wadi Khaled.
There was no immediate report of casualties, but this latest incident came just two days after border clashes in which two girls were killed and several other people wounded in Lebanon.
Both the European Union and the United States had expressed concern at the recent cross-border clashes, just hours before the latest violence broke out..