The order to declare a closed military zone, signed by Gaza area commander General Dan Harel, means that only residents, workers in essential services and journalists will be allowed to enter the 21 settlements due to be dismantled from August.

"Updated information exists regarding additional groups of Israelis that are moving towards the Gaza Strip in an attempt to provide backup for the rioters and further inflame the situation," an army spokesman said.

"A further escalation of this course of action taken by radical elements may have grave effects on the region."

The move follows general outrage over the tactics of radical youths, who have been amassing inside the main Gush Katif settlement bloc.

A Palestinian teenager was left critically ill after being attacked by a mob of Israelis on Wednesday who had earlier taken over a three-storey building in a Palestinian village.

"The attack on the Palestinian teenager is a barbaric, wild and heartless act," Israeli Prime Minister Ariel Sharon told Thursday's Yediot Aharonot newspaper as 16-year-old Khaled el-Astal lay seriously ill in hospital.

"What is happening in front of our eyes is not a battle over disengagement from Gaza, but a battle over the image of the state," he added. "This is not a situation I will allow to continue. The situation is not lost, and we will win."

Astal was attacked by the same youths who had also taken over a house in the Palestinian village of Mawassi, which lies inside Gush Katif.

Security forces later stormed the house, making around 40 arrests, after Sharon had given instructions to use an iron fist to tackle extremist opponents of his disengagement plan.

Televised footage of the violence, which also captured some of those holed-up in the house pelt soldiers with stones and yoghurt, has appalled many Israelis and was seen as a major own goal to the settlers' cause.