JERUSALEM -- Palestinian militants detonated a bomb next to an Israeli army patrol along the border with Gaza on Tuesday, killing one soldier and wounding three others in the first serious clash since a cease-fire went into effect more than a week ago.
"We will respond, but there is no point in elaborating," Barak said in comments released by his office.
There was no claim of responsibility for the attack.
The incident jolted the calm that has largely prevailed since Israel ended a devastating three-week offensive on Jan. 17. Since withdrawing its troops, Israel has threatened to strike hard at any violations of the truce.
Heavy gunfire was audible along the border in central Gaza and Israeli helicopters hovered in the air firing machine gun bursts, Palestinian witnesses said. An Israeli jet broke the sound barrier and set off a loud sonic boom over Gaza City not long afterward, possibly as a warning, but there were no further reports of Israeli retaliation.
The Israeli military said the bomb targeted an Israeli patrol near the border community of Kissufim.
It was not clear if the bomb had been planted after the cease-fire took hold or whether it was an older device.
Not long after the bombing, a 27-year-old Gaza farmer was killed by Israeli gunfire along the border several miles away, said Dr. Moaiya Hassanain of Gaza's Health Ministry. Two other Palestinians were wounded. The military had no immediate comment and it was unclear if the two incidents were related.
Israel closed its crossings into Gaza to humanitarian aid traffic after briefly opening them Tuesday morning. Gaza border official Raed Fattouh said Israeli officials informed him the closure was due to the attack.
Israel and Gaza militants have been holding their fire since Israel ended its offensive, which was aimed at halting rocket fire from the territory. Israel announced a unilateral cease-fire on Jan. 17, and that was followed by a similar announcement from Gaza militants.
In the days immediately following the cease-fire there was shelling by Israeli gunboats and some gunfire along the border - including the killing of two men Palestinian officials identified as farmers - but there were no serious clashes until Tuesday.
Although there was no claim of responsibility, Mushir al-Masri, a Hamas leader, said Israel was to blame for continuing to fire into Gaza. Al-Masri said his group had not agreed to a full cease-fire but only to a "lull" in fighting.
"The Zionists are responsible for any aggression," he said.
Egypt is currently trying to negotiate a longer-term arrangement to allow quiet in the coastal territory of 1.4 million people, which has been ruled by the Islamic militants of Hamas since June 2007.
Israel wants an end to Hamas rocket attacks and guarantees that Hamas will be prevented from smuggling weapons into Gaza from Egypt. Hamas has demanded that Israel and Egypt reopen Gaza's border crossings, which have been largely closed since Hamas took power. The crossings are Gaza's economic lifeline.
The Israeli offensive killed 1,285 Palestinians, more than half of them civilians, according to records kept by the Palestinian Center for Human Rights. Thirteen Israelis, including three civilians, were also killed during the fighting.
Associated Press writer Ibrahim Barzak in Gaza City contributed to this report.