U.S. Secretary of State John Kerry criticized Israel in candid remarks  caught on an open microphone between television interviews Sunday.
Kerry was heard talking about Israeli soldiers killed in Gaza to a State Department official identified as Jonathan Finer just before appearing on the "Fox News Sunday" political talk show.
"I hope they don't think that's an invitation to go do more," Kerry says. "That better be the warning to them."
A frustrated Kerry then says: "It's a hell of a pinpoint operation, it's a hell of a pinpoint operation," in apparent frustration over the civilian toll in the Israeli operation.
The Palestinian death toll topped 500 on Sunday  as Israel pressed a ground invasion into the densely packed Gaza Strip after two weeks of aerial bombardment.
"We've got to get over there," Kerry is heard saying on the Sunday recording. "I think, Jon, we ought to go tonight. I think it's crazy to be sitting around. Let's go."
When confronted over the remarks by Fox host Chris Wallace, Kerry reiterated Israel's right to "self-defense."
"I think it's very, very difficult in these situations, obviously very difficult," Kerry said.
"It's tough to have this kind of operation. I reacted obviously in a way that ... anybody does with respect to young children and civilians.
"But war is tough, and I've said that publicly and I've said it again. We defend Israel's right to do what it is doing in order to get at those tunnels," Kerry said.
Kerry then urged Hamas to "step up and be reasonable and understand that you accept the ceasefire, you save lives."
Kerry traveled to Cairo early Monday f or crisis talks on Gaza, following President Barack Obama's call for an "immediate ceasefire" between Israelis and Palestinians in the conflict.