Asked during a media conference call if the cease-fire was over, Lt. Col. Peter Lerner, an Israeli military spokesman, said: "Yes. We are continuing our activities on the ground."

In the border region between Israel and Gaza, plumes of black smoke could be seen rising from residential areas in the coastal enclave as Israel resumed shelling early Friday morning.

A 72-hour Gaza cease-fire crumbled only hours after it began Friday, with at least 70 Palestinians killed by Israeli shelling, Israel saying one of its soldiers may have been abducted and both sides trading accusations of violating the truce.

Gaza officials say at least 1,600 Palestinians, mostly civilians, have been killed and 8,700 wounded. Sixty-two Israeli soldiers have been killed and more than 400 hurt. Two Israeli civilians and one Thai citizen have also been killed.

Israel launched its offensive in Hamas-dominated Gaza on July 8, unleashing air and naval bombardments in response to what it said was a surge in cross-border rocket attacks. Tanks and infantry pushed into the territory of 1.8 million people on July 17.

Palestinian President Mahmoud Abbas said a joint Palestinian delegation, including Hamas and Islamic Jihad, would travel to Cairo on Saturday for cease-fire talks, despite the renewed fighting.

Egypt re-iterated later Friday its invitation to Israeli and Palestinian delegations, saying it is "still in place" despite the 72-hour humanitarian truce breaking down.

The cease-fire was to be followed by Israeli-Palestinian negotiations in Cairo on a longer-term solution. The collapse of the cease-fire does not mean the diplomatic efforts are dead, but violence near the Gaza-Egypt border is preventing some of the movement of officials who were set to participate.

The attempted 72-hour break in fighting, announced by U.S. Secretary of State John Kerry and U.N. Secretary-General Ban Ki-moon, was the most ambitious attempt so far to end more than three weeks of fighting.

"Israel claims that a soldier was captured to hide their crimes and to divert the public opinion to speak of the captured soldier instead,” he said. "We do not have any information about a captured soldier."

Hamas, however, denied capturing a soldier. Osama Hamdan, a senior Hamas official, told France 24 news channel Friday that Israel had concocted the story to “cover up its crimes.”

Two Israeli soldiers were killed in the clash, the Israel military confirmed.

The Israeli military said one of its soldiers, 23-year-old Hadar Goldin, was feared captured during a clash inside a tunnel in the southern Gaza Strip. The military said that during the clash one of the fighters it encountered tried to detonate an explosive belt and that there was an exchange of gunfire. Lerner said an "extensive effort" was underway to locate the soldier.

Trading blame

About two hours after Friday’s truce went into effect, Israeli tanks and artillery opened fire in the southern Rafah area.

The Israeli military had no immediate comment on the shelling, but media reports said it began after Hamas fighters exchanged fire with Israeli soldiers on a mission to destroy tunnels that are used by Palestinians to smuggle goods in and out of Gaza, which has endured a seven-year economic blockade by Israel. Armed fighters, including some from Hamas and Islamic Jihad, have also used the tunnels to launch attacks on Israeli targets.

Eight rockets and mortar bombs were fired from Gaza at Israel, the military said, adding that one was intercepted by the Iron Dome missile defense system and seven hit open areas.

In a White House press conference Friday, President Barack Obama condemned the Israeli soldier's capture and underlined U.S. support for Israel's "right to defend itself" but expressed concern over Israel's offensive in Gaza.

"Part of the reason why we've been pushing so hard for a cease-fire is precisely because it's hard to reconcile Israel's legit need to defend itself with our concern for those civilians," Obama said.

In a phone call between Israeli Prime Minister Benjamin Netanyahu and Kerry hours earlier, the prime minister said, “Palestinians had blatantly breached the humanitarian cease-fire and attacked our soldiers.”

Kerry issued a statement after the call condemning "in the strongest possible terms" Hamas' killing of two Israeli soldiers today, and demanding the group "immediately and unconditionally release the missing Israeli soldier."

Hamas' operation was an "outrageous violation of the cease-fire," Kerry said.

The U.S. Senate on Friday approved an emergency bill to give Israel $225 million in additional funding for its Iron Dome missile defense system, which it has used with relative success to counter Hamas rockets.

"They're running out of Iron Dome missiles to protect themselves," said Sen. Lindsey Graham, R-S.C., according to The Hill. "We are with you. Here are the missiles."

Washington agreed on Wednesday to also allow Israel to replenish its dwindling munitions with a local U.S. arms stockpile.

For Hamas' part, the group pinned blame for violating Friday's cease-fire on Israel.

“We, in Hamas, hold the [Israeli] occupation fully responsible for the breach of the 72-hour ceasefire agreed on,” a statement by the group read.

Hamas said that its clashes with Israeli troops were an extension of earlier fighting that Israel had instigated before the cease-fire commenced.

“The enemy’s army moved its vehicles and soldiers, and advanced within the territory of the Gaza Strip in the depth of Rafah, at 6:30 a.m., and it advanced 2,400 [meters] so the Palestinian resistance repelled the infiltrating forces and engaged in clashes with them at 7 a.m. in the morning,” the group said. “The clashes continued after the time the truce came into force.”

Hamas said its commitment to the cease-fire was “linked” to Israel’s commitment.

While a statement attributed to the spokesman for the U.N. secretary-general condemned “the reported violation by Hamas of the mutually agreed humanitarian ceasefire,” it conceded that the “UN has no independent means to verify exactly what happened.”

Al Jazeera and wire services. Nick Schifrin contributed to this report from Gaza.