Any resistance to this occupation now will be legitimate and accepted by all Palestinians against the occupiers

Ahmed Abdel-Rahman Yasser Arafat aide

The move follows an Israeli cabinet decision to seize and hold on to parts of the West Bank "as long as terror continues".

It is a response to Tuesday's suicide bomb attack in Jerusalem that left 19 Israelis, several of them children, dead - the worst attack in the city for six years.

Palestinian officials have denounced the new Israeli policy and said it is likely to provoke more violence.

'Murderous acts'

Israeli forces rolled into the West Bank towns of Jenin, Qalqilya and Nablus on Wednesday morning.

Witnesses say clashes broke out in the Jenin refugee camp, the focus of a controversial Israeli operation in April.

One report describes Israeli troops in Jenin bringing in mobile homes on the back of trucks - apparently preparing for an extended stay.

The army later withdrew from Nablus after arresting a number of suspected militants.

Prime Minister Ariel Sharon has made it clear Israel may seize more towns.

"Additional acts of terror will lead to the taking of additional areas," a statement issued by his office early on Wednesday said.

The new policy signals "a change in the way Israel responds to murderous acts," it said.

Before Tuesday's suicide attack, Israel had talked about short-term operations to arrest wanted militants and confiscate weapons.

An Israeli security official said the new policy would essentially amount to bigger and longer incursions.

He said that to fully reoccupy Palestinian towns, the army would have to call up reservists and take on responsibility for civil administration - something it is not yet prepared to do.

'Occupiers and invaders'

Palestinians have reacted with fury.

Israel's cabinet approved a series of military operations

"The Israeli decision to reoccupy Palestinian territories and settle inside them represents an open invitation to the Palestinian people and to all Palestinian factions to practise all sorts of resistance against the occupation," Ahmed Abdel-Rahman, an aide to Yasser Arafat, told the Reuters news agency.

"Any resistance to this occupation now will be legitimate and accepted by all Palestinians against the occupiers and the invaders of Palestinian sovereign areas," he said.

In Ramallah - the West Bank town which is home to Mr Arafat - residents are reported to be stocking up on food and medicines in case the Israeli Army returns.

Against terrorist acts, Israel will respond by capturing territories of the Palestinian Authority - these territories will be held by Israel as long as terror continues

Israeli Government statement

Mr Sharon said on Tuesday that the nail bomb attack on the bus in Jerusalem - which also left 50 people injured - was an argument "stronger than any words" against Palestinian statehood.

The militant group Hamas claimed responsibility for the attack and has promised further action.

President Bush is expected to outline an American vision for the setting up of a Palestinian state - and possibly a timetable for that to happen - and his spokesman said he would not be put off by the latest "act of terror".

Bastions

The Israeli army has recently been carrying out regular raids on towns such as Jenin, which are viewed as bastions of militants and future suicide bombers.

Those raids followed the withdrawal from Palestinian towns under Operation Defensive Shield, which ran from 29 March to 10 May.

At least 1,403 Palestinians and 530 Israelis have been killed since the Palestinian uprising began in September 2000.