Siniora accused the Israelis of "a clear violation of Lebanese sovereignty and an unacceptable provocation" after Beirut claimed the man had been abducted from inside Lebanon.

The Israeli military said it had detained a man crossing the border with Lebanon.

"During an IDF (Israeli army) activity a suspect was identified crossing the Blue Line into Israel. He was arrested inside Israel and has been taken in for questioning," an Israeli army spokeswoman said.

She declined to give the suspect's nationality but said he was not Israeli.

However, a Lebanese security source told AFP the Israeli soldiers had abducted a man they named as Fadi Ahmad Abdel Aal inside Lebanon and "brought him inside Israel with them.

"The soldiers made an incursion of 100 metres (yards) to reach agricultural land where they found the shepherd," the source said on condition of anonymity.

The UN peacekeeping force said it had been informed of the incident by the Israeli military.

"UNIFIL has been informed by the IDF that they have apprehended a Lebanese citizen and we are in contact with them in order to resolve the situation as soon as possible," UNIFIL spokeswoman Yasmina Bouziane told AFP.

Israeli forces have in the past seized Lebanese shepherds along the border and taken them for questioning over possible links with Hezbollah militants. They are usually released after questioning.

UNIFIL is tasked with monitoring the ceasefire between Hezbollah and Israeli forces following a 34-day war in July-August 2006 in which more than 1,200 Lebanese were killed, mostly civilians.