A one-ton bomb dropped by an Israeli war plane killed a senior Hamas official on Thursday, in addition to killing two of his wives and four of his children, during the sixth day of an Israeli air attack aimed at key Hamas targets.

The bomb killed 49-year-old Nizar Rayan, who had close ties to Hamas's military wing and who was considered one of the organization's top decision makers.

Medics said 18 others also died in the attack, including nine of Rayan's 12 children and his four wives. The Muslim faith allows men to have up to four wives.

"We are trying to hit everybody who is a leader of the organization, and today we hit one of their leaders," Israeli Vice Premier Haim Ramon said in a television interview.

The bomb set off other explosives stored inside Rayan's four-storey apartment building, sending it crumbling to the ground.

Israeli jets also hit several Palestinian government offices, including Gaza's parliament building, in the latest strikes.

Bombs also fell on tunnels used to smuggle goods underneath Gaza's border with Egypt.

More than 400 people -- including 34 children -- have been killed and 1,700 wounded since the Israeli strikes began.

Israeli war planes have flown about 500 sorties over the Gaza Strip, in retaliation against militants who have been firing rockets into Israel since a six-month truce collapsed in December.

Meanwhile, Israel ramped up preparations for a possible ground assault against Hamas targets in Gaza, amassing more troops at the border.

Soldiers gathered at the edge of the densely-populated strip of land, along with dozens of tanks. Israel's government also approved the call-up of 9,000 reservists.

"The infantry, the artillery and other forces are ready," said Israeli military spokesperson Maj. Avital Leibovich. "They're around the Gaza Strip, waiting for any calls to go inside."

Top Israeli officials have rebuffed pleas from the international community to stop the violence.

Speaking in Paris Thursday in advance of a visit to the Middle East by French President Nicolas Sarkozy, Israeli Foreign Minister Tzipi Livni said the offensive has crippled Hamas operations in Gaza.

"We affected most of the infrastructure of terrorism in Gaza Strip and the question (of) whether it's enough or not will be according to our assessment on a daily basis," Livni said.

Earlier this week, Israeli Prime Minister Ehud Olmert rejected a French proposal for a two-day break in the violence.

However, Olmert did tell U.S. Secretary of State Condoleeza Rice and other leaders that Israel would not agree to a truce unless international monitors would be available to enforce such an agreement, according to Israeli government officials who would not be named.

Calls for an immediate ceasefire have also come from the Quartet -- the U.S., E.U., UN and Russia -- who joined together to attempt to broker a peace agreement in 2002.

Although it has rejected the temporary ceasefire, Israel has agreed to allow 2,000 tons of food and medical supplies into Gaza Wednesday.

With files from The Associated Press