Blasts kill Palestinian militants

The blast tore through Hamas' third-floor office

Eight Palestinians, including two senior members of the Islamic militant group Hamas, have been killed by Israeli fire in the town of Nablus in the West Bank.

The Israeli people should know that they will pay the price, and our blood is not cheap

Sheik Yassin, Hamas spiritual leader

Jamal Mansour and Jamal Salim, both senior Hamas activists, were among those killed when Israeli helicopters struck a research office belonging to the organisation.

Witnesses said two children who had been playing outside the seven-storey building at the time were also among the dead.

Sheik Ahmed Yassin, Hamas' spiritual leader, warned that the Israeli people would "pay the price" for the deaths, adding that Palestinian blood was not cheap.

Hundreds of Palestinians staged angry protests on the streets of Nablus following the attack - which was the most deadly by Israel since the Palestinian uprising began last September.

Mansour: Hamas denies he was involved in military operations

Israel said the strike was a preventative measure aimed at senior Hamas activists behind a wave of anti-Israeli attacks, engaged in planning more suicide bombings.

"Israeli Defence Forces struck at a senior group of Hamas members which committed terror attacks in the past, and were engaged in other terror activities," said a statement from Prime Minister Ariel Sharon's office.

It added: "The army acted in order to prevent acts of murder and sabotage against Israelis," the statement added.

Speaking to the BBC Mr Sharon's spokesman, Ranaan Gissin said the loss of innocent civilian life was regretted, but he suggested the children may have been used as "human shields".

A Hamas spokesman told the BBC that both leaders belonged to the group's political wing and had no connection to military operations.

The United States - usually Israel's staunchest ally and defender - issued an outspoken condemnation of the operation.

"This attack represents an escalation, is highly provocative, and makes efforts to restore calm much more difficult," said State Department spokesman Charles Hunter.

In Gaza, two other Palestinians are also thought to been killed by Israeli fire, one an Islamic militant and the other a policeman.

Twenty-three-year-old Hamoud Nasri el-Mahdoun was killed at a border crossing with Israel while on a "military mission" for Islamic Jihad.

Later Palestinian policeman Mohammed Assad al-Hassani, 21, was killed as he was driving near the settlement of Netzarim.

'Active defence'

Israel's security forces have confirmed that the dead Hamas activists were on their list of Palestinian targets.

Israel has just begun to strike at the interests of the Palestinian Authority

Uzi Landau, Israeli Public Security Minister

The Islamic movement is violently opposed to Israel. It has carried out a series of bombings in recent years, and has stepped up its campaign during the recent violence.

The BBC's Jeremy Cooke says the latest incident appears to be a continuation of what Israel describes as a policy of "active defence".

Israeli forces have targeted and killed a number of Palestinians belonging to both Hamas and Islamic Jihad groups over recent weeks.

Blockade tightened

Earlier on Tuesday, Israeli forces tightened their blockade of Palestinian towns in the West Bank amid increasing concern over militant activity.

Two projectiles are thought to have been fired at the building

In Jerusalem, security was stepped up in public places after police said they had information that Palestinians were planning a bombing campaign in the city.

Israel Radio said the army had also intensified its presence around the West Bank cities of Ramallah, Bethlehem and Tulkarem to try to prevent Palestinian militants from crossing into Israel.

Palestinian radicals have already vowed to avenge the deaths of six activists killed in an explosion at a warehouse in the West Bank on Monday, and a subsequent Israeli helicopter attack on the police headquarters in Gaza City.

The latest violence threatens to undermine international peace efforts to end confrontations in which more than 600 people, mostly Palestinians, have been killed.

A ceasefire brokered by the US in mid-June has failed to take hold.