Israeli troops rolled into northern Gaza Tuesday, sparking the deadliest fighting with local militants in more than a year and killing at least 19 Palestinians, including two killed in the evening in an airstrike in the northern Gaza Strip.
Another 45 people were injured, 12 of them seriously, hospital officials said.
Taher Nounou, the spokesman for the Hamas administration in de facto control of Gaza, announced three days of mourning.
In the latest strike, a group of militants belonging to the Mujahidin Brigades of the Fatah movement were launching homemade rockets from northern Beit Hanoun at the Israeli town of Sderot, when they were struck by a missile fired from an Israeli aircraft.
In addition to the deaths in the incursion, a foreign volunteer in an Israeli kibbutz (agricultural commune) bordering the Strip was shot dead by a Palestinian sniper.
The 20-year-old volunteer from Ecuador was shot in the back while working in a potato field near the Israel-Gaza border fence.
Among the Palestinian dead was the son of 62-year-old Mahmoud al- Zahar, a senior Hamas hardliner. Mahmoud al-Zahar's son Hussam, 23, was taking part in Tuesday's fighting when killed, residents said.
Hussam is Mahmoud al-Zahar's second son to die in Israeli- Palestinian violence. Another son, Khaled, was killed in a September 2003 Israeli airstrike on his house. Al-Zahar and his now paralyzed wife were both moderately injured in that airstrike, which had come a day after double suicide bombings in Jerusalem and south of Tel Aviv.
"This crime is the fruit of US President George Bush's visit to the region," al-Zahar told reporters at the morgue after identifying his son's body.
"He encouraged the Israelis to kill our people," he said, in apparent reference to unconfirmed Israeli media reports that said Bush had given Israel the "green light" to target Hamas' political leadership.
Israel holds Hamas ultimately responsible for the increase in daily cross-border rocket and mortar attacks since the Gaza takeover.
Hamas will "respond to this crime in the suitable time and way," al-Zahar threatened. Referring to his son's death, he said: "There is no difference between martyrs. All of them fall for the sake of the land."
Palestinian President Mahmoud Abbas of the Fatah movement also condemned the Israeli incursion as a "massacre" and "slaughter," while his spokesman Nabil Abu Rudeineh said in the West Bank city of Ramallah that "these brutal acts weaken the positions of the Palestinian negotiator and harm the peace process."
Witnesses said a number of Israeli tanks and bulldozers drove some 400-500 metres into northern Gaza around dawn, reaching up to the eastern outskirts of Gaza City. They began levelling makeshift structures and agricultural fields, from which militants have been firing rockets into Israel.
An Israeli military spokeswoman in Tel Aviv said heavy fighting erupted when Palestinian militants confronted the force with mortars and semi-automatic fire. She said the Israel Air Force launched at least three airstrikes at a vehicle and squads of militants during the fighting.
"We are trying to target those armed terrorists in order to defend our people," Major Avital Leibovich told Deutsche Presse-Agentur dpa.
Hospital officials said nearly all the dead were militants, most of them members of Hamas' armed wing, but a 67-year-old civilian bystander was also among those killed.
The death toll was the highest in Israeli-Palestinian fighting since a November 8, 2006 Israeli artillery strike at the northern Gaza town of Beit Hanoun killed 18 Palestinians. Most of the dead at that time were civilians, many of them members of the same family. Israel at the time blamed a "technical fault" for missing its target, a squad of militants firing rockets into Israel.
The latest escalation in fighting came after Abbas' and Israeli Prime Minister Ehud Olmert's negotiators met in Jerusalem Monday for the first serious peace talks in seven years.
The negotiations were due to have started immediately after a November 27 peace conference in Annapolis, Maryland, but were delayed amid a dispute over Israeli settlement construction.
But after last week's visit by President Bush, the sides announced they would finally start talks on the "core issues" of their mutual conflict, including Jerusalem, borders and refugees.
Meanwhile, militants of the Islamic Hamas movement's armed wing, al-Qassam brigades, fired four homemade "Qassam" rockets at Sderot Tuesday evening.
It was the first direct rocket attack in a year by Hamas militants on the town, which has been subject to such attacks by other militant groups loyal to Hamas, mainly the Islamic Jihad and the Popular Resistance Committees.
Israeli Radio reported that one of the rockets that hit Sderot wounded at least five Israelis. A state of emergency was declared in the town, the report said. ( Dpa )