Unguarded comments from John Kerry on Israel's military operations in the Gaza Strip have been caught on a microphone he did not appear to know was recording.
The US secretary of state has given his firm backing to Israel in recent days as it expanded its offensive against Hamas.
But in comments recorded while he waited to give an interview to Fox News he appeared less certain.
"It's a hell of a pinpoint operation, it's a hell of a pinpoint operation," Mr Kerry can be heard to say in a phone conversation with an aide named John, in an apparent reference to an Israeli offensive that saw 13 soldiers killed.
"We've got to get over there. Thank you John. I think John, we ought to go tonight. I think it's crazy to be sitting around."
When pressed by host Chris Wallace what he meant by the remarks, Mr Kerry responded: "It's tough to have these kind of operations. I reacted obviously in a way that anybody does with respect to young children and civilians.
"War is tough. I've said that publicly and I've said it again. We defend Israel's right to do what it is doing. We support Israel's right to defend itself against rockets that are continuing to come in."
Hours earlier, America's top diplomat had told ABC News that Israel was "under siege".
"Israel is under siege by a terrorist organisation that has seen fit to dig tunnels and come through those tunnels with handcuffs and tranquiliser drugs, prepared to try to capture Israeli citizens and take them back to hold them hostage," Mr Kerry said.
"No country could sit by and not take steps to try to deal with people who are sending thousands of rockets your way."
Palestinian health workers say 400 people have died in Israel's fortnight-long offensive, with the United Nations saying the majority are civilians.