U.S. President Barack Obama has called for the release of an Israeli soldier who is believed to have been abducted by Hamas militants on Friday as renewed violence broke a 72-hour humanitarian ceasefire that had begun Friday.

The Israeli military said two soldiers were killed and an infantry officer was feared captured during fighting in Rafah in the southern Gaza Strip.

An hour after a ceasefire between Israel and Hamas had begun, gunmen emerged from one or more Gaza tunnels and opened fire at Israeli soldiers, with at least one of the militants detonating an explosives vest, said Israeli army spokesman Lt. Col. Peter Lerner.

Hadar Goldin, a 23-year-old 2nd Lt. from the central Israeli town of Kfar Saba, was apparently captured in the ensuing mayhem while another two Israeli soldiers were killed.

"We suspect that he has been kidnapped," Lerner said.

In a news conference on Friday afternoon, Obama addressed the fighting and called for Goldin’s captors to release him immediately.

“We have and I have unequivocally condemned Hamas and the Palestinian factions that were responsible for killing two Israeli soldiers and kidnapped a third,” the U.S. president told reporters.

“If they are serious about trying to resolve this situation, that soldier needs to be unconditionally released as soon as possible.”

Abducted Israeli soldier Hadar Goldin

The Israeli military says 2nd. Lt. Hadar Goldin, 23, died during a clash near the Israel-Gaza border on Friday. (YNet News/AP)

The flare-ups in the violence came less than two hours into a 72-hour ceasefire that had been agreed upon by both Israel and Hamas. The breakdown of the truce and the apparent capture of the Israeli soldier set the stage for a major escalation of the 25-day-old conflict, which has already devastated large swaths of the coastal area and killed at least 1,500 Palestinians, mainly civilians, according to Palestinian officials. Israel has lost 63 soldiers and three civilians.

Obama said the fighting represents a setback in U.S.-mediated diplomatic efforts to end the conflict and called upon Hamas to ensure its militants respect any truce that’s established.

“It's going to take some time,” he said. “I think it's going to be very hard to bring a ceasefire back again.”

An hour after Friday's ceasefire started, gunmen emerged from one or more Gaza tunnels and opened fire at Israeli soldiers, with at least one of the militants detonating an explosives vest, said Israeli army spokesman Lt. Col. Peter Lerner.

Israeli Prime Minister Benjamin Netanyahu, meanwhile, had told Kerry in a telephone conversation that Palestinian militants had "unilaterally and grossly" violated the ceasefire.

"Israel will take all necessary steps against those who call for our destruction and perpetrate terrorism against our citizens," Netanyahu told Kerry, according to a statement from the prime minister's office.

Moussa Abu MarzoukHamas's deputy leader, told Al-Arabiya news channel from Cairo that the movement's military wing carried no military operations after 8 a.m. Friday, when the truce came into force.

A Hamas spokesman, Fawzi Barhoum, would neither confirm nor deny the capture, saying it was being used — along with news that two Israeli soldiers were killed in the Rafah area — as a cover for a "massacre."

The UN urged both sides to recommit to the ceasefire in a Friday morning statement.

Search for soldier sparks more violence

Israeli forces, backed by heavy tank fire and airstrikes, moved deeper into southern Gaza late Friday in search of Goldin.

In Gaza's southern Rafah area, the military urged residents in phone calls to stay indoors as troops advanced.

Israel Palestine Gaza conflict

A brief reprieve from intense fighting in Gaza was broken just hours after it began as four Palestinians were killed by Israeli tank shells Friday, according to the Palestinian health authority. (Suhaib Salem/Reuters)

"We are under fire. Every minute or so, tanks fire shells," said Ayman al-Arja, 45, a resident of the area.

The Israeli military said the heavy shelling in Rafah that followed was part of operational and intelligence activity designed to locate Goldin.

Prime Minister Stephen Harper’s office condemned Hamas in a statement released Friday, saying it is "solely to blame" for any further loss of life in the conflict.

"Canada is appalled that Hamas, only hours in to the ceasefire, has yet again blatantly violated this agreement by abducting an Israeli soldier," the PMO said its statement on the situation.

"The people of Gaza have suffered greatly under Hamas's reign, and it is high time that their needs are put first over their rulers' blind ambition."

Gaza cleanup begins

Soon after the ceasefire went into force, Gaza's residents took advantage of the truce to return to their homes, many of which had been destroyed in the fighting.

During its daily press briefing, a UN spokesman said the humanitarian crisis on the ground in Gaza is nearing the “breaking point.”

Near a main road in the heavily bombarded Gaza district of Shijaiyah, less than a mile from the Israeli border, residents surveyed extensive damage.

Gaza cleanup amid ceasefire

Palestinians in Gaza City began efforts to clean up as a 72-hour ceasefire took hold, even as fighting continued in the southern town of Rafah. (Mohammed Salem/Reuters)

​Basem Abul Qumbus returned to find his three-story home — in which he had invested tens of thousands of dollars — uninhabitable. Tank shells had punched a hole in the ceiling of one bedroom and a wall had collapsed into the kitchen.

"The work of all those years is gone," he said, as he struggled to salvage flour from bags that had been torn apart by shrapnel. Food supplies are running short in the blockaded coastal territory in the war's fourth week.

In the southern town of Khan Younis, residents searched for bodies in the rubble of their homes as rescuers and volunteers carried away corpses, some charred, on makeshift stretchers.

Nidal Abu Rjeila found the charred body of his disabled sister on the ground on the side of the road, her wheelchair flipped upside down. He said her body had been there for five days.

"I tried to reach human rights groups and the Red Cross, but no one was answering me," he said while lying next to her body.

Abducted soldier's father speaks out

Israeli soldiers

Some Israeli ground forces withdrew from the Gaza Strip on Saturday, two Israeli television stations reported, after the military said it was close to achieving its main war goal of destroying cross-border tunnels. (Jack Guez/AFP/Getty)

A longtime friend of Goldin's said he is engaged to get married and that he studied at a religious Jewish seminary in the West Bank settlement of Eli. Goldin has a twin brother who is also in the military on the Gaza front-lines, said the friend, who spoke on condition of anonymity because he did not have the family's permission to discuss Goldin's personal details with the media.

The soldier's father, Simha Goldin, is a Tel Aviv University professor.

"We want to support the military in the fighting against Hamas in Gaza. We are sure the military will not stop before it turns over every stone in Gaza and returns Hadar home safe and sound," the father said in a brief statement to media outside his home.

Diplomatic negotiations to resume in Egypt

Egypt issued a statement early Friday calling on the Western-backed Palestinian Authority and Israel to send negotiation teams to Cairo to discuss "all issues of concern to each party within the framework of the Egyptian initiative."

Egypt had put forth a ceasefire proposal a week after fighting began last month. Israel accepted the proposal, but Hamas, which deeply mistrusts Egypt following last summer's overthrow of an Islamist government in Cairo, rejected it.

Hamas has demanded the lifting of an Israeli and Egyptian border blockade imposed on Gaza in 2007 when the Islamic militant group seized power, as well as the release of Palestinians rounded up in the West Bank in June following the killing of three Israeli teenagers.

In recent weeks Turkey and Qatar, which have warmer ties to Hamas but are at odds with Egypt, have tried to help broker a ceasefire agreement, with no results.

It's not clear whether other nations will attend the Egypt talks, and aides to Kerry said Egypt will ultimately decide who will participate. A Hamas official in Qatar said Hamas and Islamic Jihad officials would be participating. Israel will not meet directly with members of either group because it considers them terrorist organizations.

With files from CBC News and Reuters