Israeli troops have raided a prison compound in the West Bank town of Jericho, killing one Palestinian guard.
The Israeli army is demanding the handover of a militant leader it holds responsible for the assassination of a government minister in 2001.
Ahmed Saadat of the Popular Front for the Liberation of Palestine is refusing to come out, as are five comrades.
Palestinian officials have been talking about freeing Mr Saadat under certain conditions, a move Israel opposes.
One of a group of PLFP inmates held in Jericho reportedly told a Palestinian MP by phone that all six prisoners were in their cells when the Israeli forces entered the compound.
The troops were calling on the prisoners over loudspeakers to come out of their cells, the MP said.
Mr Saadat was arrested in connection with the killing of Tourism Minister Rehavam Zeevi by PFLP gunmen in 2001, an attack which itself was to avenge the assassination of Mr Saadat's predecessor by Israel.
Reports from the scene said 50 jeeps and three tanks pushed into the oasis town in the Jordan valley, as two helicopters hovered overhead.
The army later brought a large armoured bulldozer to the prison.
Mr Saadat is normally guarded by British and US prison guards, in addition to Palestinian jailers, but the British wardens had withdrawn for what they described as security reasons.