NEW YORK — Israel suffered through its bloodiest day since the conflict began, even as diplomatic efforts aimed at ending Israeli-Hezbollah hostilities faltered.

At least 15 people were killed Sunday by Hezbollah attacks on Israel, 12 in a rocket attack on the Upper Galilee.

Sunday’s Katyusha attack on Kibbutz Kfar Giladi, where reserve soldiers are believed to be headquartered, is the most deadly barrage into Israel since the war with Hezbollah began in mid-July.

Several people were also wounded by the rocket, which landed on a crowd in the kibbutz.

There were indications that the reserve soldiers killed had ignored a warning siren to seek shelter.

“The soldiers who were hit were lying in a shaded area at the entrance to Kfar Giladi Cemetery,” one reserve officer who was there at the time of the strike told Ha’aretz. “They were lying on mattresses and resting.

“We had already been there a week and there had been a siren almost every hour, and it had already started to become routine. We joked among ourselves that wherever we were was a safer place to be than Kiryat Shmona.”

Later Sunday in Haifa, at least three people were killed after several rockets struck the Israeli port city.

More than 30 people were wounded after the Hezbollah-fired rockets struck Haifa, Israel’s third largest city.

At least five people were pulled from a building that partially collapsed after it was struck by a rocket.

Despite the attack, Haifa Mayor Yona Yahav vowed his city’s residents would persevere.

“We can overcome and prevail over any difficulties,” he told CNN on Sunday.

Israel responded with an attack on rocket launchers in the southern Lebanese town of Kana, which was believed to be the origin of the rockets that hit Haifa.

Earlier Sunday, Israel bombed other suspected Hezbollah targets in southern Lebanon and Beirut.

In addition to Sunday’s airstrikes, which reportedly killed five civilians, at least 10 Hezbollah fighters were reportedly killed in clashes with Israeli soldiers in southern Lebanon. Several Israeli soldiers were wounded in the fighting.

Meanwhile, a leading Lebanese politician said Lebanon rejects a U.N. Security Council draft resolution aimed at ending fighting between Israel and Hezbollah.

Lebanon’s Parliament speaker, Nabih Berri, said Lebanon would refuse to accept the resolution because it allows Israeli troops to remain in Lebanon. Syria also said it rejects the resolution.

Shimon Peres, Israel’s deputy prime minister, told CNN’s Wolf Blitzer on Sunday that Israel would wait until the proposal, which does not call for the return of the two Israeli soldiers Hezbollah kidnapped last month, is finalized before reacting.

The draft resolution calls for Hezbollah to stop all military operations and for Israel to stop its push into Lebanon, wording which could allow Israel to keep troops in Lebanon as a defensive measure.

The proposal agreed to by France and the United States this weekend would allow Israel to strike back if Hezbollah were to break a cease-fire.

The U.S. secretary of state, Condoleezza Rice, said Sunday that the Security Council could vote on the resolution in the coming days.