JERUSALEM An agreed three-day cease-fire collapsed Friday only 90 minutes after it went into effect when Hamas militants attacked Israeli soldiers working to demolish tunnels in the Gaza Strip, killing two and apparently kidnapping a third, the Israeli army said.

The renewal of hostilities, and especially the presumed capture of the missing army officer, dimmed hopes for an early halt to the Hamas-Israel conflict, now it is fourth week.

Israeli forces, backed by tank-shelling and airstrikes, pushed deeper into the southern Gaza Strip in a search for the soldier near the town Rafah, raising the prospect of a further expansion of the Israeli offensive against Hamas.

At least 70 Palestinians were reported killed in the Rafah area, and at least 140 throughout the Gaza Strip.

President Barack Obama and U.N. Secretary-General Ban Ki-moon called for the immediate release of the soldier, reflecting concerns that his disappearance could trigger a further escalation of fighting and an even higher toll of civilian casualties.

More than 1,600 Palestinians, most of them civilians, have been killed since Israel launched its offensive against Hamas on July 8, according to Gaza health officials. Sixty-three Israeli soldiers have died, along with three civilians killed by rocket strikes in Israel. An estimated 400,000 Palestinians in Gaza have fled their homes, according to the United Nations.

"I want to make sure that they are listening," Obama said, referring to Hamas. "If they are serious about trying to resolve this situation, that soldier needs to be unconditionally released, as soon as possible."

Obama said Washington would continue working for a cease-fire, but it would "very hard" to reach one "if Israelis and the international community can't feel confident that Hamas can follow through on a cease-fire commitment."

Ban said the apparent abduction was a "grave violation" of the cease-fire and called for the "immediate and unconditional release of the captured soldier."

Israeli Prime Minister Benjamin Netanyahu told Secretary of State John Kerry, who had worked to arrange the halt in fighting announced Thursday, that Hamas had "brazenly violated the humanitarian cease-fire" and "will bear the consequences of their actions."

Israel's security cabinet met to discuss Israel's next moves, but no decisions were announced.

The suspected capture of the officer, which the army said came during an attack on troops working to uncover a Hamas tunnel near Rafah, raised questions about whether Hamas had agreed to continued Israeli operations against the tunnels during the cease-fire.

An army statement said that "initial indications suggest" that the officer had been "abducted by terrorists" when militants attacked troops working to find and destroy a Hamas tunnel at 9:30 a.m. local time, an hour and a half after the cease-fire went into force.

Lt. Col. Peter Lerner, an army spokesman, said a militant suicide bomber who emerged from a tunnel shaft detonated his explosives as gunmen opened fire, and two soldiers were killed.

Findings at the scene indicated that the officer, identified as 2nd Lt. Hadar Goldin, 23, of the Givati infantry brigade, was taken by the militants, Lerner said. He said he had no information on whether the officer had been wounded.

The army statement said the military was "conducting intelligence efforts and extensive searches in order to locate the missing soldier." Israel Radio said that special forces, along with armor, infantry and engineering units, were joining the search.

Israeli witnesses in the border area reported heavy artillery barrages and firing from Israeli helicopter gunships in the area of Rafah in the southern Gaza Strip.

Palestinians reported more than 40 people killed and 200 injured in the Israeli shelling and airstrikes, including at least one members of an ambulance crew who died when their vehicle was struck. Clashes between Israeli forces and militants were reported east of Rafah, and Israeli troop movements were reported in other locations. Militants fired fresh volleys of rockets into Israel, but no casualties were reported.

The armed wing of Hamas, the Qassam Brigades, did not immediately claim it was holding the soldier, though Musa Abu Marzouk, the second-ranking political leader of Hamas, told the Turkish Anadolu news agency that the officer had been captured and two soldiers killed before the cease-fire went into effect.