The offensive into the town of Beit Hanoun began just hours after Israeli forces ended a three-day-long incursion into Khan Younis refugee camp in southern Gaza aimed at halting mortar and rocket attacks by militants against nearby Jewish settlements.

The violence, a week before Palestinians elect a successor to Yasser Arafat, posed a particular challenge to Abbas, who has called for a resumption of peace talks with Israel under a US-backed "road map" and an end to bloodshed.

Witnesses said about 50 Israeli tanks and armored personnel carriers moved into the northern Gaza Strip. The raid began after two rockets hit the southern Israeli town of Sderot, wounding a woman.

"In light of the continuous launching of Qassam rockets which terrorize communities inside Israel, the Israel Defense Force is operating in the launching areas ... to reduce the attacks," a military communiqu said.

There were no immediate reports of fighting or casualties.

The military statement gave no timeframe for the latest raid. Israel has promised to pull its forces out of Palestinian towns at least 24 hours before voting begins on Jan. 9.

Meanwhile Mahmud Abbas, the frontunner in next week's election for a new president of the Palestinian Authority told AFP in Gaza city that the spate of raids by the Israeli army into the Gaza Strip are designed to undermine next week's vote.

"We are very sorry about this Israeli escalation," Abbas said in an interview on Saturday night.

"The target of this escalation is put an obstacle in the way of the Palestinian election. The international community must be very careful about what is going on -- the democratic process here is being put in danger."

Abbas, who is hoping to succeed the late Yasser Arafat as president in the January 9 election, has been campaigning in Gaza since Friday against a backdrop of violence across the territory.