* Gaza truce collapses, Israel hunts for tunnels
* Obama, Ban demand release of apparently captured soldier
* Almost 1,600 Palestinians killed in Gaza offensive (Updates with rockets intercepted over Israel, Washington approves Iron Dome funding)
By Nidal al-Mughrabi and Ori Lewis
GAZA/JERUSALEM, Aug 2 (Reuters) - The armed wing of the Palestinian Islamist group Hamas said on Saturday it had no clear indication on the whereabouts of an Israeli soldier that Israel has accused it of abducting in the Gaza Strip, adding he may have been killed during an ambush.
Israel said Second Lieutenant Hadar Goldin, 23, who went missing on Friday, had been abducted by Hamas gunmen. It declared a planned 72-hour Gaza ceasefire over, saying Hamas militants breached the truce soon after it took effect.
The ceasefire lasted only about 90 minutes early on Friday. Israel resumed shelling, killing at least 150 Palestinians and wounding hundreds of others, hospital officials said.
A latest signal that the ceasefire was over came at daybreak when Israel's Iron Dome interceptor system shot down two militant rockets over the Tel Aviv area and a third over Beersheba.
Israeli Prime Minister Benjamin Netanyahu called his security cabinet into special session and warned Hamas and other militant groups they would "bear the consequences of their actions". No announcement was made after the meeting.
But a statement by Hamas's armed wing said it had no contact with militants who were operating in the southern Gaza Strip where Israel said Goldin went missing.
"We have lost contact with the group of fighters that took part in the ambush and we believe they were all killed in the (Israeli) bombardment. Assuming that they managed to seize the soldier during combat, we assess that he was also killed in the incident," the statement said.
The planned 72-hour break in fighting announced by U.S. Secretary of State John Kerry and U.N. Secretary-General Ban Ki-moon hours before it was due to take effect early on Friday was the most ambitious attempt so far to end more than three weeks of fighting.
At least 1,592 Gazans have been killed since the start of hostilities on July 8 when Israel launched its drive to halt militant rocket fire on its territory from the coastal enclave by unleashing air and naval bombardments. Tanks and infantry pushed into the territory of 1.8 million people on July 17.
Sixty-three Israeli soldiers have been killed, and Palestinian rockets have killed three civilians in Israel.
U.S. President Barack Obama called for the soldier's unconditional release and said it would be tough to reinstate a truce after the day's events.
"I think it's going to be very hard to put a ceasefire back together again if Israelis and the international community can't feel confident that Hamas can follow through on a ceasefire commitment," he told a White House news conference.
Obama said he has been in constant contact with Netanyahu about the situation, and added that more needed to be done to protect Palestinian civilians.
In a boost to Israel, the U.S. Congress approved $225 million in emergency funding for Iron Dome, sending the measure to Obama to be signed into law. The House of Representatives approved the funding by a 395-8 vote late on Friday, several hours after the Senate passed it unanimously.
Kerry said he had asked Qatar, which is close to Hamas, and Turkey to help free the soldier.
"We have urged them, implored them, to use their influence to do whatever they can to get that soldier returned," a senior State Department official told reporters travelling with Kerry. "Absent that, the risk of this continuing to escalate, leading to further loss of life, is very high."
Turkey's foreign minister said his country would do its best to help, but that reinstating the truce should be the priority.
Ban also condemned Hamas's reported violation of the ceasefire and demanded the release of the soldier.
The ceasefire, which began at 8 a.m. (0500 GMT) on Friday, had prompted Palestinian families to trek back to devastated neighbourhoods where rows of homes have been reduced to rubble.
It was to be followed by Israeli-Palestinian negotiations in Cairo on a longer-term solution.
A senior Egyptian Foreign Ministry official said the talks would still begin on Sunday, and that Cairo "expects the two sides to cease fire before the launch of negotiations".
SEARCHING FOR TUNNELS
The Israeli military said that 90 minutes into the truce, militants attacked soldiers searching for tunnels in the southern Gaza Strip used to infiltrate fighters into Israel.
"Out of a tunnel access point or several, terrorists came out of the ground. At least one was a suicide terrorist who detonated himself. There was an exchange of fire," said Lieutenant-Colonel Peter Lerner, a military spokesman. Two of the soldiers were killed.
"The initial indication suggests that a soldier has been abducted by terrorists during the incident," he told reporters. Mark Regev, a Netanyahu spokesman, said Hamas was responsible for the attack.
Hamas's armed wing said they would not cease fire against Israeli forces that carry incursions inside Gaza.
"It is possible that any (Israeli) invading force would clash with one of our ambushes and that will definitely cause a confrontation," the group said in its statement.
Israel also said there was no truce to uphold. Asked if the ceasefire was over, Lerner replied: "Yes. We are continuing our activities on the ground."
Kerry said the international community "must now redouble its efforts to end the tunnel and rocket attacks by Hamas terrorists on Israel and the suffering and loss of civilian life".
The truce had left Israeli ground forces in place in the Hamas-dominated Gaza Strip and a military spokeswoman had said operations would continue to destroy the tunnels through which the Islamist group has menaced Israel's southern towns and army bases.
Israeli officials have long voiced concern that militants would try to capture a soldier or an Israeli civilian. In 2011, Israel released more than 1,000 Palestinian prisoners in exchange for Gilad Shalit, a soldier snatched by Hamas five years earlier.
Amid strong public support in Israel for the Gaza campaign, Netanyahu had faced intense pressure from abroad to stand his forces down. International calls for an end to the bloodshed intensified after shelling that killed 15 people sheltering in a U.N.-run school in Gaza's Jabalya refugee camp on Wednesday.
Hamas, isolated in an Arab world concerned about the rise Islamist militancy, is seeking an end to Israel's blockade of Gaza. It also wants a hostile Egypt to ease restrictions at its Rafah crossing with the territory imposed after the military toppled Islamist president Mohamed Mursi last year. (Writing by Ori Lewis, Additional reporting by Patricia Zengerle and David Lawder in Washington,; Editing by Mohammad Zargham and Simon Cameron-Moore)