Jerusalem - Israel hit back at the launch sites of the Qassam rocketslast night with artillery fire and air strikes. War planes broke the sound barrier over the Gaza Strip to warn Palestinians of the consequences if the rocket attacks continued.

Israeli forces had been placed on high alert along the border with the Gaza Strip as guerrillas launched rockets into Israeli territory in what the militants called an "initial response" to the killing of two of their most-wanted militants earlier.

Israeli soldiers shot dead Louay Sa'adi, one of the most-wanted Islamic Jihad military commanders, and Majed Ashkar, his lieutenant, in an early-morning raid yesterday on the West Bank town of Tulkarem.

They also arrested five other members of a cell, which was said to have been planning a suicide bombing inside Israel within the next few days. One of them, Mohammed Mueen, gave himself up after troops threatened to destroy his house if he did not surrender. Palestinian witnesses said soldiers burned another house.

The Palestinian Authority, which has been maintaining an edgy ceasefire, condemned Israel's operation as counter-productive. Saeb Erekat, the Palestinians' chief negotiator, said: "This threatens the ceasefire. It will add to the violence and counter-violence."

Israeli security sources said Islamic Jihad had actually increased its military activity, especially in the northern West Bank, since the ceasefire was agreed at the Sharm el-Sheikh summit in February. Many of its fighters, the sources claimed, did not feel commitment to a period of calm.

Islamic Jihad vowed to avenge the killings with "blood for blood". Abu Abdullah, a spokesman for the Gaza military wing, said: "Our retaliation for this crime will be unprecedented. We will not stand handcuffed while the blood of our fighters is shed."

Security sources blamed the Tulkarem cell for two suicide bombings that killed a total of 10 civilians this year inside Israel.

The first was outside the Stage disco in Tel-Aviv in February, the second at a shopping mall in the resort town of Netanya in July.

Israel also suspected Sa'adi of planning two abortive attacks, including a double suicide bombing in Jerusalem in May.

An Israeli spokesman said that Majed Ashkar, 26, opened fire on troops from a vehicle as they crept up on the house where Louay Sa'adi, 26, was hiding. The soldiers shot back Neighbours showed reporters a trail of blood that led inside the house.

Despite the ceasefire, Israel has arrested or killed more than 600 Palestinians on the West Bank since mid-September, when Hamas militants launched 40 Qassam rockets from Gaza into southern Israel. One of them was found unexploded last week in the grounds of the Negev ranch of Ariel Sharon, the Prime Minister. Hamas called it a warning that no Israeli was safe.

Israel broke off diplomatic contacts with the PA last week after three young settlers were killed near Hebron. A summit meeting between Mr Sharon and Mahmoud Abbas, the Palestinian President, scheduled for earlier this month, has been postponed indefinitely.

James Wolfensohn, the international envoy who has been trying to revive the Palestinian economy, has accused Israel of foot-dragging on the issue of Palestinian freedom of movement since it evacuated Gaza in September.

In a letter, leaked yesterday to the daily Ha'aretz, the former World Bank chief said Israel was acting "almost as though there had been no withdrawal, delaying making difficult decisions and preferring to take difficult matters back into slow-moving subcommittees".