Ehud Barak, the leader of Israel’s center-left Labor Party, turned down a proposal on Monday by the prime minister-designate, Benjamin Netanyahu of the conservative Likud Party, to join a broad governing coalition.

The refusal dealt a further blow to Mr. Netanyahu’s efforts to forge a unity government. His meeting on Sunday with Tzipi Livni, the leader of the centrist Kadima Party, ended without agreement.

“The voters’ verdict has sent the Labor Party into the opposition,” Mr. Barak said. “I told Netanyahu that we will serve as a responsible, serious and constructive opposition.” In the Feb. 10 election, his party won only 13 of the 120 seats in Parliament.

Mr. Netanyahu has six weeks to form a government and says he will press on in his pursuit of national unity. Ms. Livni and Mr. Barak both said they would meet with him again if asked.

Kadima narrowly beat Likud in the elections, making it the largest party in Parliament by one seat, but Likud commands a larger bloc, which includes religious and right-wing parties. Mr. Netanyahu could quickly form a government with these partners but would prefer a broader, less hawkish coalition.

His success will depend on how committed he is to the religious and rightist parties that endorsed him to become prime minister, and what, if anything, he has promised them. Both Ms. Livni, now the foreign minister, and Mr. Barak, the defense minister, have said they will head into opposition before legitimizing a government that includes elements of the far right.

Meanwhile, the departing prime minister, Ehud Olmert, suspended a senior official of the Defense Ministry, Amos Gilad, on Monday from his role as Israel’s main liaison with Egypt. Egypt is trying to broker a long-term truce between Israel and the militant group Hamas, which controls Gaza. The talks are on hold until Mr. Gilad’s replacement is announced, the prime minister’s office said.

The suspension was the culmination of a public spat that began when Mr. Olmert said this month that there could be no deal on a Gaza cease-fire that did not include Hamas’s release of Cpl. Gilad Shalit, a captured Israeli soldier. Mr. Gilad publicly criticized the demand as “madness,” saying that it risked damaging relations with Egypt.

But the Defense Ministry and some members of Mr. Olmert’s cabinet expressed dismay at the suspension. A Hamas spokesman, Fawzi Barhoum, said the suspension “shows that the Zionist occupation government has no intention of reaching an agreement” on either a truce or a prisoner exchange.

In a separate development, Amnesty International issued a report on Monday accusing Israel and Hamas of misusing foreign-supplied weapons to “attack civilians” during the recent 22-day Gaza war.

Amnesty called on the United Nations to impose a comprehensive arms embargo on both parties, and it urged the Obama administration to suspend American military aid to Israel.

The Israeli Foreign Ministry said in a statement that the report ignored “the basic fact that Hamas is a terror organization” and that while the Israeli military never intentionally attacked civilians during its offensive, Hamas deliberately used Palestinian civilians as human shields.

Mr. Barhoum of Hamas also rejected the Amnesty report, saying that it was “unbalanced and unfair” and that it did not distinguish between “the real criminal and the victim.”

Amnesty issued another report this month that accused Hamas of a “campaign of deadly retribution” against Palestinians accused of collaborating with Israel.

Amnesty said at least two dozen men had been shot dead, and scores of others shot in the legs or otherwise abused.