Jerusalem ? Israeli forces killed five Palestinians in the West Bank and one in the Gaza Strip on an unusually bloody day of fighting Saturday that also left two Israelis injured in a rocket attack.

The clashes were the deadliest this year in the West Bank and marked a further unraveling of a 5-month-old cease-fire agreement in Gaza, where Israel staged an airstrike on a car near the source of the rocket fire.

The flare-up reflected a stalemate over a U.S.-backed proposal by Palestinian Authority President Mahmoud Abbas to extend the Gaza truce accord to the West Bank. Israeli Prime Minister Ehud Olmert, in talks with Abbas, has refused to consider a broader cease-fire until militant groups in Gaza stop violating the truce with rocket attacks.

In turn, the militants insist that the strikes from Gaza are justified as long as the Israelis target their West Bank comrades in operations such as the one that set off Saturday’s chain of killing and retaliation.