Israeli undercover soldiers shot and killed a top military leader of the Islamic Resistance Movement, or Hamas, today in continuing retaliation for the bombing of an Israeli tank in which four soldiers were killed Saturday, according to the Israeli military and Palestinian officials.

The Israeli military said in a statement that its soldiers went into the Palestinian-run Gaza Strip to arrest the man, identified as Riyad Abu Zeid, 32, reportedly the second-ranking leader in the Hamas military wing, Izzedine al-Qassam. Zeid "was planning to carry out a massive terror attack inside Israel," the statement said, and during the operation to arrest him, Zeid and others with him opened fire on the soldiers, who returned fire.

Zeid was taken into custody but died from his wounds en route to a hospital in Israel, the statement added.

Hamas officials called the killing an assassination. Palestinian officials and witnesses said Zeid and others were ambushed about 11 a.m. as they drove in a car on the coastal road in a central area of the Gaza Strip. Witnesses quoted by the Associated Press said Zeid's car was fired upon by undercover troops hiding in a blue van loaded with vegetables that was parked on the side of the road.

One man in the car died at the scene and an Israeli military ambulance took the driver and a passenger away, the witnesses said.

The killings brought to at least 10 the number of Palestinians killed since Saturday in the Gaza Strip. The fighting seems to herald a new offensive against Hamas, a leading proponent of suicide bombings against Israelis. The organization is headquartered in Gaza and has not come under the same intense crackdown as other Palestinian militant organizations that are stronger in the West Bank, where the Israeli army has tightened its occupation since last June.

The attacks and counterattacks come as senior Israeli and Palestinian officials have traveled to London for meetings with various international officials -- including the so-called Quartet of diplomats from the United States, Russia, the European Union and the United Nations -- to discuss Palestinian reforms, economic assistance and cease-fire plans.

Abdel Aziz Rantisi, a senior Hamas political leader, denied that Zeid was a member of the organization's military wing, saying he was a political figure. But Palestinians who knew him, and who would not be quoted by name, said he was a leading Hamas fighter.

Zeid was described as the number two man in the military wing behind Mohammed Deif, a master bomb-maker who was critically injured last September in an assassination attempt by the Israeli military. Deif was reported to have lost an eye in the Sept. 27 helicopter strike, which killed two people and injured about 30 others, including 15 children.

Deif reportedly rose to the top position in the Hamas militia after its leader, Salah Shehada, was killed last July when an Israeli F-16 dropped a one-ton bomb on a building in a crowded residential neighborhood of Gaza City, killing 14 others and injuring more than 150.

Six other Hamas activists were killed Sunday in a mysterious car explosion while inspecting an item in the trunk of a car. Hamas officials said the item was rigged with a hidden bomb that was detonated by Israeli forces; the Israeli military would not comment on the incident. Two more Palestinians were killed early today in heavy firing that occurred when 35 Israeli tanks entered Gaza City to destroy the house of a leading Hamas militant, according to reports.

Israeli Defense Minister Shaul Mofaz had promised to "strike hard at our enemy Hamas" in retaliation for the soldiers' deaths on Saturday.

Hamas leader Rantisi promised the same for Israel in an interview today. After the recent deaths, he said, the Hamas military wing "now has the permission and authorization to commit any operation they like, at any time, in any way they like."

Palestinian witnesses said the car carrying Riyad Abu Zeid and others was ambushed by soldiers in a van carrying vegetables. The Israeli army said he was killed in a firefight.