THE roar of a low-flying Israeli F16 signals that an airstrike is imminent. A blast echoes across the valley, followed by a tower of dust and smoke climbing into the sky.

Smoke billows from a Hezbollah border position. A few miles away in Marjayoun, the deep boom of Israeli shellfire can be heard as round after round lands in a valley.

The air raids and the advance of Israeli tanks and troops marked the beginning of Israel’s biggest strike across its northern border into Lebanon since it withdrew from the country in May 2000.

Ehud Olmert, the Prime Minister, his generals and Cabinet ministers emerged from an emergency session last night promising a ”severe response” to the worst, and most complex, crisis facing the new Israeli Government. Israel held Lebanon responsible for the plight of the abducted soldiers, a Cabinet statement said.

Before the expected large-scale operation to recover its troops, the Israeli army prepared to call up thousands of reservists and told residents of border towns to get into underground bomb shelters.

The Lebanese front was reopened in an echo of Gaza’s hostage crisis 160km (100 miles) south, where events began yesterday. At 2.30am Israeli aircraft dropped a 250kg (550lb) bomb on a house in Gaza in an attempt to assassinate Mohammed Deif, the leader of Hamas’s military wing. The raid killed a Hamas activist, Nabil al-Salmiah, his wife and seven of his children, who were aged four to fifteen.

Mr Deif, for more than a decade Israel’s most wanted man, was seriously injured and underwent four hours of spinal surgery. Palestinians feared that he might be paralysed.

The Israel Defence Forces (IDF) said that they hit the house after a tip about a meeting of senior Hamas leaders. The assassination attempt drew Hamas threats of revenge and provoked outrage. Akram Shanti, a neighbour, said that he was confronted by the headless body of a small girl when he rushed to help. “I was speechless by what I saw,” Mr Shanti said. “It was utterly shocking. This will not stop the violence, just make people even more angry towards Israel.”

Within hours the focus shifted to Lebanon where, as in Gaza, Israeli commanders ordered troops back into hostile territory that they had evacuated only recently, to recover abducted colleagues.

According to the Israeli version, a Hezbollah raiding party had sneaked across the border early in the morning and planted landmines on a road used by Israeli Humvees, killing three of the reconnaissance patrol and capturing two near the border town of Shtula.

“They placed explosive devices on the road, the Humvees drove on to them and then they fired an anti-tank missile,” an IDF spokeswoman said.

Simultaneously, Katyusha rockets were fired at Israeli villages and army posts in the area, injuring five civilians in what appears to have been a diversionary ploy.

The IDF refused to confirm Israeli newspaper reports that the guerrillas used a ladder to climb over the electric fence separating the two countries.

Israeli soldiers and tanks pursued the raiding party back into Lebanon, where an Israeli tank was blown up, killing four more. A fifth soldier was shot while trying to recover the stricken tank. The IDF said that a guerrilla had tried to infiltrate the western side of the border with Lebanon, but was detected and shot dead.

Mr Olmert said: “These are difficult days for the state of Israel and its citizens. There are elements in the north and south that threaten our stability and challenge our stamina and determination. They will fail and pay a dear price for their actions.”

Amir Peretz, the Israeli Defence Minister, said that Israel would “employ any means it deems fit, and the army has been instructed accordingly”.

Just as they destroyed Palestinian bridges, roads, a power station and government buildings in Gaza, including the Foreign Ministry building last night, Israeli F16s and artillery began blasting escape routes in southern Lebanon.

General Udi Adam, head of the IDF’s Northern Command, said: “All targets are legitimate. We have destroyed all the Hezbollah outposts in the border and we are now continuing to operate in depth, mainly from sea and air.” He confirmed that troops had found blood marks, suggesting that the captured soldiers were wounded, but he had “no information if they are alive”.

In Beirut the Hezbollah leader Sheikh Hassan Nasrallah said that the raid was mounted out of solidarity with the Palestinians. Cautioning that Hezbollah was prepared to repel Israeli troops, he said: “We are ready for confrontation if they want it, and they have to expect surprises.”

Times Recommends

• Mandela’s tribe threatens President Zuma
• The French empire strikes back
• Wife of cheating governor exacts a political revenge