A top Hamas leader and several members of his family were killed Thursday in an Israeli air strike in the Gaza Strip, as Israel pressed its offensive for a sixth day and a Hamas rocket destroyed part of an apartment building in the Israeli port city of Ashdod.

Nizar Rayyan, a cleric who served as a liaison between Hamas's political and military wings, had called for renewed suicide bombings inside Israel and refused to go into hiding, as other prominent Hamas members had done. His death, confirmed in a statement issued by Hamas, marked the first known killing of a senior leader of the militant group since Israel began air strikes on Saturday. Two of Rayyan's four wives and four of his 12 children were also killed after Israeli forces bombed their apartment building, Palestinian health officials said.

Israel's punishing assault on the densely populated and impoverished Gaza Strip has killed at least 410 people, including scores of civilians, and wounded more than 1,600 people, according to health officials in Gaza. Three Israeli civilians and a soldier have been killed by rocket fire from Gaza since the offensive began on Dec. 27.

Israeli forces have destroyed Hamas buildings, institutions, tunnels and military posts, prompting international calls for a ceasefire so humanitarian relief can be provided.

But, while Israeli foreign minister Tzipi Livni traveled to Europe Thursday to meet with leaders there, Israeli officials continued to reject any calls to pause their attacks. Prime Minister Ehud Olmert said Israel would fight Hamas with an "iron fist" in an effort to stop the organization from launching rockets and missiles into southern Israel.

The rocket fire continued Thursday, including the one that hit the Ashdod apartment building. No one was injured.

Israeli aircraft and naval forces attacked about 20 Hamas targets in Gaza Thursday, including a government complex, the Israeli military said.

Czech prime minister Mirek Topolanek, who holds the European Union presidency, said Thursday that he was planning to organize a diplomatic mission on behalf of the bloc to the Middle East to address the conflict there. An Israeli official said French President Nicolas Sarkozy planned to visit Jerusalem on Monday, following Livni's visit to France.

Israeli leaders have vowed to continue attacks on Hamas targets in the Gaza Strip, including possibly sending in ground forces.

At a cabinet meeting Wednesday, Israeli leaders rejected a French proposal for a 48-hour cease-fire. Israel will consider a cease-fire only when there is "a real, sustainable calm," said government spokesman Mark Regev.

"A cease-fire will give them time to rest, regroup and rearm," Regev said. "A temporary solution might sound nice, but it's a mirage. It will only blow up in our faces in a couple weeks or a month."

"We think inflicting serious blows on the Hamas military machine is a crucial ingredient in achieving that situation," Regev added. "It is possible we have to pursue military operations for a longer period before conditions are right for a sustainable calm."

In recent days, Israel has dispatched more troops and tanks to the border. It has also announced plans to call up more than 9,000 reservists to active duty. Israel, which is hoping to extract a ceasefire on its own terms, could be massing its ground forces as a pressure tactic.

A ground incursion is "an option," Regev said. "It has not yet been decided upon. But it is clearly an option we have." He stressed, however, that Israel, which pulled its forces from the Gaza Strip in 2005, has no intention of reoccupying the area.

Diaa Rashwan, an analyst on Islamist movements at al-Ahram Center for Political and Strategic Studies in Cairo, said that if the Israelis go back into the densely populated strip, they will play into Hamas's hands. Hamas "will gain the sympathy of the Arab world as well as bolster their image and strength," Rashwan said. "It will be a great day for Hamas. They will have the best of both worlds -- to be a victim and a resister at the same time."

On Wednesday, Hamas leader Ismail Haniyeh declared in a televised speech that the Islamist movement would emerge victorious.

"We tell the Palestinian people in Gaza and everywhere that you will win, inevitably. Victory is near, God willing, and it is closer than people think," Haniyeh said. Hours earlier, an Israeli airstrike destroyed his office. Israel said the office was used for planning attacks against the Jewish state.

Human rights groups warned that ground combat could lead to more civilian deaths. "Israeli forces must bear in mind that there are no 'safe' places in Gaza for civilians to seek shelter," Amnesty International said in a statement Wednesday. "The Israeli army must always choose means and methods of attack that are least likely to harm civilians."

Faced with growing calls to help suffering Palestinians, Israel announced Wednesday that it was permitting shipments of humanitarian supplies to enter Gaza. Israel controls checkpoints in the strip. More than 90 trucks carrying food and medicine crossed into Gaza on Wednesday, and more were expected Thursday.

"We say publicly, and we mean it: The innocent civilian population is not our enemy," Regev said.

French Foreign Minister Bernard Kouchner proposed a 48-hour cease-fire that would allow humanitarian groups to ferry into Gaza medicine, food, water and other basic necessities that are in short supply. The proposal was part of a broader effort, which intensified Wednesday, by the Quartet of Middle East peace mediators -- the United States, the United Nations, the European Union and Russia -- to halt the bloodshed.

White House spokesman Gordon Johndroe said President Bush spoke Wednesday to Israeli Prime Minister Ehud Olmert, who reassured Bush that Israel was targeting Hamas operatives and trying to avoid civilian casualties. When asked whether the two had discussed a timetable for a cease-fire, Johndroe said they had not discussed anything specific.

"What's more important is the goal. As I said, we all want to see an end to the violence as soon as possible," Johndroe said at a news conference in Crawford, Tex., where Bush is visiting his family ranch. "President Bush wants to see an end to the violence. Prime Minister Olmert wants to see an end to the violence. But I think from the prime minister's perspective, an end to the violence means that Hamas stops firing rockets into Israel and Israel won't have to go after the rocket launchers."

Yigal Palmor, a spokesman for the Israeli Foreign Ministry, said the cease-fire proposal "did not contain the necessary elements to make the truce permanent. It lacks a plan to enforce the cease-fire, to make sure Hamas won't shoot rockets into Israel anymore, and stop the smuggling of weapons."

"It does not contain any guarantees," he added.

Palmor said Israel was open to future diplomacy. "There is a lot of work to be done," he said.

On Thursday, Israeli Foreign Minister Tzipi Livni is scheduled to fly to Paris to meet with French President Nicolas Sarkozy to discuss ways to end the crisis.

Arab foreign ministers meeting in Egypt called on Palestinian factions to put aside their differences. In a final statement, the ministers also urged the U.N. Security Council to issue a resolution demanding an immediate cease-fire. The Security Council met in an emergency session to discuss the violence but adjourned without a vote.

The streets of Gaza City were relatively quiet Wednesday. Residents wandered around their homes, fixing windows and doors and cleaning up debris from the streets.

Tamer Mansour, 36, a local television editor who lived near Haniyeh's office, walked up the stairs of his damaged apartment complex. His apartment, filled with shattered glass and rubble, was permeated with a burning odor, he said. He and his wife cleaned up the mess. But they are afraid to live there again. A Hamas commander resides on the same block, he said.

"All of the neighbors are afraid to go home because they expect his house will be bombed," Mansour said. "We're still afraid they might bomb Haniyeh's office again."

The Israeli government has barred foreign journalists from entering Gaza. On Wednesday, after a petition by the Foreign Press Association, Israel's Supreme Court ruled that the government must allow journalists entry. It gave the government until Thursday morning to allow limited access.