JERUSALEM — Palestinian militants sprang from the ground and confronted Israeli soldiers Friday morning, as they have repeatedly in recent days. This time, Israeli officials said, one exploded a suicide belt while another unleashed machine-gun fire. This time, two Israeli soldiers were killed and the militants apparently escaped with a third.

The attack, at the start of what was supposed to be a 72-hour pause in the fighting, escalated the deadly 25-day battle between Israel and Hamas, the Islamist faction that dominates the Gaza Strip.

Israel said the attack, from under a house near the southern border town of Rafah, took place at 9:20 a.m., soon after the 8 a.m. onset of the temporary truce secured by the Obama administration and the United Nations, whose leaders squarely blamed the breakdown on Hamas.

Hamas’s account was confused. One leader was quoted claiming responsibility for the soldier’s capture, then backtracked. Others contended that the clash unfolded at 7 a.m., before the cease-fire, although Palestinian reports of fighting near Rafah came three hours later. And one said that in any case, the Hamas gunmen acted only to counter “Zionist incursions.”

What was clear was that the episode dimmed prospects for curtailing a conflict that has killed more than 1,600 Palestinians, many of them women and children, and plunged Gaza into a humanitarian crisis. Israel responded with an assault that killed 70 people and injured 350 around Rafah alone as troops sealed the area to hunt for the missing officer amid mounting pressure from Israeli politicians and the public to expand the military mission.

The deadly attack and counterattack sharpened a sense that intensive diplomacy is proving ineffective and irrelevant to the asymmetrical combat on the ground. Secretary of State John Kerry had made clear in announcing the cease-fire that Israel would be allowed to continue operating against tunnels from Gaza into its territory, something one Hamas spokesman indicated Friday was contrary to “the Palestinian understanding with mediating parties.”

The events renewed command-and-control questions about Hamas, a guerrilla group torn by rivalries and communication snags between its military and political rulers in Gaza and abroad. They also suggested neither side is ready for an exit ramp until its goals are met: for Israel, destruction of the tunnels and a halt to rocket fire from Gaza, and for Hamas, a score that can be leveraged to change the social and economic conditions of Gaza’s 1.7 million beleaguered people.

“It’s going to be very hard to put a cease-fire back together again if Israelis and the international community can’t feel confident that Hamas can follow through,” President Obama said on Friday at the White House. He called the killing of civilians in Gaza “heartbreaking” and said, “It’s possible we may be able to arrive at a formula that spares lives and also ensures Israel’s security, but it’s difficult, and I don’t think we should pretend otherwise.”

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Both Mr. Kerry and Ban Ki-moon, the secretary general of the United Nations, demanded an immediate and unconditional release of the Israeli officer. Mr. Ban described the attack as “a grave violation of the cease-fire” that called “into question the credibility of Hamas’s assurances to the United Nations.”

Israeli fears about kidnapping have been palpable since Hamas fighters used a tunnel under the border to enter Israeli territory near a kibbutz outside Gaza on July 17. Later that night, Israel launched a ground invasion to accompany the air campaign that began on July 8. Several similar attempts to infiltrate Israel have been thwarted; after one, Israel found plastic hand-ties and tranquilizers. For Hamas, which in 2006 abducted Staff Sgt. Gilad Shalit and five years later traded him for more than 1,000 Palestinian prisoners in Israeli jails, a live hostage is perhaps its most powerful weapon.

After an intense predawn battle in the Gaza City neighborhood of Shejaiya on July 20, Hamas announced that it had captured Staff Sgt. Oron Shaul and broadcast his identification number, prompting celebrations across Gaza and the West Bank. Israel later said Sergeant Shaul had been killed in action, but no remains had been recovered.

Israeli military officials said they were uncertain of the condition of the officer captured on Friday. They identified him as Second Lt. Hadar Goldin, 23, of the elite Givati Brigade. Lieutenant Goldin has a twin brother who until Friday was also fighting at the front, according to Israeli news reports, and he had proposed to his girlfriend during the war, scheduling the wedding in two months.

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His father, Simcha Goldin, said the family was confident the Israeli military would “not stop under any circumstances until they have turned over every stone in Gaza and have brought Hadar home healthy and whole.”

Israel’s military censor informed The New York Times that material related to the missing officer had to be submitted for review, the first such notification in more than six years. International journalists must agree in writing to the censorship system in order to work in Israel. The Times did not send the censor a draft of this article before publication, but summarized over the phone its biographical references to Lieutenant Goldin.

The attack near Rafah brought to 63 the number of Israeli service members slain; two citizens and a Thai farmworker have also been felled by rocket and mortar fire. The military said more than 60 rockets had been launched by 8 p.m. Friday from Gaza, bringing the total during the conflict to 3,025.

Lt. Col. Peter Lerner, an Israeli military spokesman, said the Givati force had been working to decommission a tunnel under a home inside Gaza more than an hour into the cease-fire when at least two Palestinians emerged from another shaft. “One came out shooting after the other one blew himself up,” Colonel Lerner said. “We were in defensive positions. They clearly abused the situation to carry out the attack, under the cover of the humanitarian window.”

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Israel sent text messages to area residents to remain in their homes as forces rushed farther into Rafah, bombarding it from the ground and air to block the captors’ escape.

Safa, a Gaza-based news agency that has run a live blog during the war, first reported artillery fire in Rafah at 9:55 a.m. The Health Ministry spokesman announced new fatalities there at 10:10 a.m. The Qassam Brigades, Hamas’s military wing, issued a statement hours later, denying that the clash occurred after the cease-fire, but also suggesting it may not have truly accepted the terms.

“We emphasize that any Zionist forces violating our liberated land would be subject to our holy fighters and a legitimate target,” the statement said.

Separately, Fawzi Barhoum, a Hamas spokesman in Gaza, said, “According to the Palestinian understanding with mediating parties, it is important for the resistance to defend our people, and itself, in the case of any renewed Israeli incursions.”

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Mkhaimer Abusaada, a political scientist at Al-Azhar University in Gaza City, said that the military wing may have been purposely defying the political team’s accession to the agreement, or at least saying that if the Israelis were allowed to keep destroying tunnels, Hamas should be permitted to try to stop them.

“It’s definitely a mess,” he said. “I think we’re going to see much worse days than those that are behind us.”

The escalation was strong and sustained, with reports in Rafah of airstrikes and heavy artillery shelling past midnight, as Israel’s top ministers met for hours to consider next steps. Colonel Lerner said the operation “now has three components, not two: it’s rockets, tunnels and an abduction now.”

Daniel Nisman, a former combat soldier who now runs a Tel Aviv geopolitical security company, said Israeli troops are taught that preventing an abduction is the highest priority, even if it means risking a captive soldier’s life by firing at a getaway vehicle. Protocol changed after Sergeant Shalit’s capture, Mr. Nisman said, “so a low-level commander on the ground can act” without awaiting orders, which had delayed action in that case.

“It’s to prevent a strategic setback that would ultimately impact the entire county,” he explained. “It sounds terrible, but you have to consider it within the framework of the Shalit deal. That was five years of torment for this country, where every newscast would end with how many days Shalit had been in captivity. It’s like a wound that just never heals.”

But some Israeli analysts noted that Sergeant Shalit, who was then a corporal, was taken from Israeli territory during a calm period, while Lieutenant Goldin should be considered a prisoner of war, a potential cost of any military campaign.

Still, Michael Herzog, a retired general and Israel-based fellow of the Washington Institute for Near East Policy, said any cease-fire discussions would be off the agenda until more information emerges about the officer’s situation.

“Right now, decision-making is more tactical by nature, until we have a better feel or clue of what exactly happened to him — is he really in their hands or not,” Mr. Herzog said.

“This complicates everything,” he added. “I don’t think we are in for any more so-called humanitarian cease-fires unless we see on the ground that they are holding their fire.”