A march on Gaza's settlements by thousands of demonstrators fizzled out on Wednesday night after security forces blocked the way, but Israeli police said they arrested about 300 people trying to slip into the occupied strip overnight.

Settlement evacuation is scheduled to begin in mid-August, but officials are increasingly worried about giving ultranationalists any more time to mount disruptive protests and foment resistance among a hard core of Gaza settlers.

"I definitely weigh positively the possibility of moving up the date set ahead of time," Vice Prime Minister Ehud Olmert told Israel Radio. "The developments of the past days have signaled a direction in nature of the clashes and protests."

A government source said Sharon, also concerned about a surge of Israeli-Palestinian bloodshed, might discuss possible changes in the pullout timetable with Secretary of State Condoleezza Rice, who begins a visit to the region on Thursday.

Sharon, who has billed his plan as "disengagement" from conflict with the Palestinians, originally planned evacuation of Gaza's 21 settlements to begin on July 22 but delayed it to avoid a Jewish mourning period that starts this weekend.

Any attempt to bring forward the withdrawal would further antagonise religious settlers and their supporters who believe Jews have a biblical birthright to Gaza, where 8,500 Jews live in isolation from 1.3 million Palestinians.