GAZA CITY, Gaza Strip ? Palestinian militants blew up an Israeli army base at the Gaza-Egypt crossing Sunday by sneaking more than a ton of explosives through a tunnel, killing five Israeli soldiers and wounding five in the largest Palestinian attack in the month since Yasser Arafat’s death.

Hitting back, Israeli helicopters fired at least five missiles at targets in Gaza City early today, witnesses said. There were no reports of casualties. One missile started a fire at an abandoned metal workshop, while the other target was an empty house near the Islamic University, they said.

Palestinian militants from Ezz el-Dein Al-Qassam brigades, the Hamas military wing, and militants from Fatah Hawks, a militia linked to the Fatah movement, talk during a joint news conference in Gaza City. Militants from the two groups blew up an Israeli army base at the Gaza-Egypt crossing Sunday by sneaking more than a ton of explosives through a tunnel, killing five Israeli soldiers.

Also Sunday, imprisoned Palestinian uprising leader Marwan Barghouti declared in a letter that he would throw his support to mainstream candidate Mahmoud Abbas in a Jan. 9 election to replace Arafat, dropping out of the race. The move rids Abbas of his strongest rival and wards off what could have been a split in the leading Fatah faction of the PLO.

For his part, Abbas, who has stepped in as interim Palestinian leader until the elections, apologized to Kuwaitis for Palestinian support of Saddam Hussein during the 1990-1991 Gulf War — his latest gesture to mend fences with Arab nations offended by Arafat.

“Yes, we apologize for what we have done,” he said after arriving in Kuwait, responding to reporters’ questions about many Kuwaitis’ long-standing demands for an apology.

In the violence along the Gaza-Egyptian border, the military said in a statement early today that five soldiers were killed and five were injured, including two seriously, in the explosion. The statement said two Palestinians charged the base and opened fire after the blast, and soldiers shot them dead.

Palestinians said one attacker was killed and the other escaped. The blast collapsed several structures at the crossing and damaged others.

The attack was another sign that the lull in violence after Arafat’s death on Nov. 11 was over.