Yasser Arafat's Fatah movement has joined a truce announced earlier in the day by other militant groups. Hamas and Islamic Jihad announced a three month halt on attacks against Israelis. The groups moved ahead of Fatah, which was hamstrung by internal fighting.
Fatah now says it will halt all military operations, in going along with an Egyptian initiative which called for a six month cease-fire.
The three militant groups had negotiated the truce last week. The timing of Sunday's announcements was a surprise, since Palestinian leaders had said they didn't expect a truce until Monday at the earliest.
Hamas and Islamic Jihad say Israel has to meet some demands for the cease-fire to hold, such as stopping targeted killings of militants.
Israel is skeptical of the truce, saying the groups may use the down time to plan more attacks.
As part of the agreement the Israeli military says its forces have started pulling out of northern Gaza. Israeli military sources say the pullout is in keeping with an agreement to hand responsibility for security in Gaza to the Palestinians.