The United States has "no evidence" to believe North Korea's claims that last week's nuclear test involved a hydrogen bomb, the State Department said Monday.

"No additional information we've gained since last week have led us to a different conclusion that we had last week, which is that there is no evidence that this was a hydrogen bomb test that the North claimed it was," State Department spokesman John Kirby said.

"But we're still doing a lot of spade work on this. I don't know how much longer that's going to take, but nothing's changed about our initial sense of it after it happened," he said.

Following Pyongyang's H-bomb claims, the White House expressed strong skepticism, saying "initial analysis is not consistent" with the North's claims. U.S. nuclear experts have expressed doubts, saying its yield was too low to believe that such an advanced bomb, way more powerful than conventional nuclear weapons, had been detonated.

The U.S. has said it will work with the international community to come up with best responses to the test. The U.S. also flew a B-52 nuclear bomber fly over South Korea in a show of force designed to send a warning to North Korea and reassure its ally of Washington's security commitment.

Kirby said that the U.S. has been working actively with allies and partners.

"Our ambassador to Seoul, Mark Lippert, has obviously been in constant communication with South Korean leaders throughout all this. Our Assistant Secretary Daniel Russel has also been in touch with counterparts and of course as I read out last week, the secretary himself made some calls on this," he said.

"From a diplomatic perspective, we've been engaged. I think it's safe to say that we will stay engaged," he said. "We want to continue to work with the international community through the U.N. to try to get at a robust set of measures to continue to hold the North accountable for provocative actions." (Yonhap)