(Updates with more quotes, context)
By Steve Holland and Mark Felsenthal
WASHINGTON, Aug 1 (Reuters) - President Barack Obama said on Friday that it will be difficult to achieve a ceasefire in Gaza after the latest truce fell apart but he strongly defended Secretary of State John Kerry for his failed diplomatic drive.
Obama's comment at a White House news conference suggested the limited impact that diplomacy is having as Israel tries to dismantle Hamas tunnel networks in Gaza and the Palestinian militants fight back.
Gaza officials say at least 1,555 Palestinians, mostly civilians, have been killed and 7,000 wounded. Sixty-three Israeli soldiers have been killed and more than 400 hurt. Three civilians have been killed by Palestinian rockets in Israel.
"It's going to be hard to get there," Obama told a White House news conference of efforts to reach a ceasefire. "I think that there's a lot of anger and there's a lot of despair. And you know, that's a volatile mix."
Obama called on Hamas and Palestinian factions to release a captured Israeli soldier as a precondition for resolving tensions between Israel and Palestinians and said efforts would be made to re-establish a ceasefire.
"If they are serious about trying to resolve this situation, that soldier needs to be unconditionally released as soon as possible," he said.
Kerry came under heavy criticism in Israel earlier this week for his latest attempt at a ceasefire after some Israelis viewed his bid for an immediate pause in fighting as basically trying to stop Israel from dismantling the Hamas tunnels.
Obama, who has had a testy relationship with Israeli Prime Minister Benjamin Netanyahu made clear he took a dim view of the criticism his top diplomat had faced.
"When I see John Kerry going out there and trying to broker a ceasefire, we should all be supporting him," Obama said. "There shouldn't be a bunch of complaints and second-guessing about, 'well, it hasn't happened yet,' or nitpicking before he's had a chance to complete his efforts."
The Israeli-Palestinian conflict has tested American diplomacy and laid bare the limits of Obama's ability to influence events in the Middle East.
Pressed on whether the United States had lost its influence in the world under his presidency, Obama dismissed what he called a "common theme" of criticism about his handling of foreign policy.
"Apparently people have forgotten that America, as the most powerful country on Earth, still does not control everything around the world," he said.
An ABC News/Washington Post poll this week said 39 percent of Americans approve of Obama's handling of the situation in Israel and Gaza while more than half, 52 percent, disapprove.
Despite American leadership, he said, "There's still going to be tragedies out there and there's going to be conflicts and our job is to just make sure that we continue to project what's right." (Additional reporting by Roberta Rampton; Editing by Peter Cooney and Sandra Maler)