Tony Blair said Thursday that a "credible" truce proposal being studied by Israel could lead to an immediate cease-fire between Israel and Hamas in the Gaza Strip, Reuters reported.

"Over the last few days the details have been discussed and worked on and I think actually there is plainly now a credible plan to stop if people want to stop," Blair, a Middle East envoy and former British prime minister, told reporters after meeting French Foreign Minister Bernard Kouchner.

Sources told Reuters that he proposal includes a yearlong, renewable cease-fire, the withdrawal of all Israel forces within 5 to 7 days, and the immediate opening of all Gaza border crossings, backed by international guarantees they would stay open.

Israeli Foreign Minister Tzipi Livni was departing Thursday night for Washington to discuss a cease-fire with Secretary of State Condoleezza Rice. The Bush administration has been working swiftly, on its second-to-last work day before President-elect Barack Obama's inauguration, to craft an agreement on American support for Egyptian-led truce mediation efforts. U.S. and Israeli officials say the U.S. would provide technical support and expertise to prevent Hamas from rearming.

U.N. Secretary-General Ban Ki-Moon said that Israel was to make an important decision on the possible cease-fire, but noted that it would take "a few more days" to work out the technical details of the truce.

Despite talks of a cease-fire, Israeli artillery shells struck the U.N. headquarters in the Gaza Strip on Thursday, setting a food warehouse ablaze and drawing a sharp rebuke from the visiting U.N. chief who called it an "outrage."

Israeli Prime Minister Ehud Olmert said Hamas militants opened fire from the U.N. compound. "It is absolutely true that we were attacked from that place, but the consequences are very sad and we apologize for it," he said. The military said it was still investigating.

Secretary of State Condoleezza Rice called the strike an "unfortunate incident" and said the Israelis had told her it was a mistake.

"There is great concern on their part," she said. "It was, they say, an error that it happened."

The intense assaults Thursday seemed to reflect an extra push to pressure Hamas negotiators into making concessions on a cease-fire and punish the militant group as much as possible before any end to hostilities.

An earlier Israeli airstrike killed Interior Minister Said Siam, a key figure in Hamas who oversaw thousands of security agents, Hamas TV said. A top aide, Siam's brother and his brother's family also were killed.

"We are talking about a key person in terms of logistics in the field, and also in the political sense," said Bassem Zbeidy, a Hamas expert in the West Bank.

He said Siam's death was a "huge loss for Hamas," but noted that the movement is easily capable of generating new leaders, often more radical than their predecessors.

Israel launched the offensive on Dec. 27 in an effort to stop militant rocket fire from Gaza that has terrorized hundreds of thousands of Israelis. It says it will press ahead until it receives guarantees of a complete halt to rocket fire and an end to weapons smuggling into Gaza from neighboring Egypt.

Rocket fire has fallen off dramatically but not ceased and on Thursday the military reported 14 firings.

The international community does not recognize Hamas' government.

The Associated Press contributed to this report.