Palestinian militants detonated a bomb that killed an Israeli soldier patrolling near Gaza on Tuesday and Israel responded with an airstrike, straining the fragile cease-fire on the eve of a visit by President Barack Obama's new Mideast envoy.

The violence jolted the calm that has largely prevailed since Israel ended a devastating three-week offensive in Gaza on Jan. 17. Since withdrawing its troops, Israel has threatened to retaliate hard for any violations of the truce.

Israeli Defense Minister, Ehud Barak called an urgent meeting of Israel's top defense officers after the bombing. "We will respond, but there is no point in elaborating," Barak said, shortly before the airstrike. Barak talked with Prime Minister Ehud Olmert after the meeting, but no details of the discussion were released.

The Israeli military dismissed earlier reports that the explosive might have been an old land mine triggered accidentally. The military said it had determined that the bomb was activated by militants, but it would not give details.

The blast also wounded three Israeli soldiers and triggered a brief battle when Israeli troops briefly crossed the border in search of the attackers. Later, Hamas said one of its militants was wounded in an Israeli airstrike.

The violence — a day before the new U.S. Mideast envoy, George Mitchell, was due in Israel — underscored the difficulty Obama faces as he tries to get Israeli-Palestinian peace efforts back on track.

Gazans are struggling to resume normal life after the fighting, and as international donors discuss how best to help the territory rebuild. Gaza's Hamas leader said Tuesday the group — which is boycotted as a terrorist organization by the U.S. and European Union — would not try to claim any of the reconstruction funds, an announcement that appeared aimed at clearing the way for money to start flowing.

The announcement from Ismail Haniyeh, who remains in hiding because of fears he could be assassinated by Israel, appeared directed at donors who concerned their funds could end up in Hamas' hands.

"Our aim now is to ease the suffering of our people and to remove the aftermath of the aggression in Gaza," the statement said. "Therefore we emphasize that we are not concerned to receive the money for rebuilding Gaza and we are not seeking that."

After Tuesday's bomb blast, heavy gunfire was heard along the border in central Gaza and Israeli helicopters hovered in the air firing machine gun bursts, Palestinian witnesses said. An Israeli jet set off a loud sonic boom over Gaza City not long afterward, possibly as a warning.

Hamas said an airstrike wounded one of its militants as he rode a motorcycle in the southern Gaza town of Khan Younis. Residents said Israeli tanks and bulldozers have also entered the area where the roadside bombing took place and were tearing up some vacant land — apparently to prevent it from being used to stage attacks.

The Israeli military said the bomb targeted an Israeli patrol near the border community of Kissufim. There was no claim of responsibility.

Not long after the bombing, a 27-year-old Gaza farmer was killed by Israeli gunfire along the border several miles away, according to Dr. Moaiya Hassanain of Gaza's Health Ministry. Two other Palestinians were wounded. The military had no comment, and it was unclear if the two incidents were related.

Later Tuesday, Palestinians said a tank shell hit a house near the border in southern Gaza. There were no reports of casualties. The military had no immediate comment.

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 for the deadly bombing, 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 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. Local experts believe the offensive caused some $2 billion in damage.

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.