U.S. Secretary of State John Kerry on Thursday said it would be "diplomatic malpractice" not to test Iran's willingness to comply with international demands over its nuclear program.

Kerry's comments Thursday in Tokyo were his first on the subject since Israeli Prime Minister Benjamin Netanyahu delivered a United Nations speech warning the U.S. and the West against trusting Iran.

Kerry said the U.S. would not take any Iranian offers at face value. He added that Iran will have to prove it's willing to come clean about its nuclear program.

Kerry said he does not believe Netanyahu was criticizing U.S. efforts to engage Iran, but rather warning that Iran and its new leader, President Hassan Rohani, might not be serious. He said the U.S. agrees with Netanyahu's assessment.

In an interview aired on Wednesday, Netanyahu told NBC that Iran is developing intercontinental ballistic missiles that could reach the U.S., whose "sole purpose is to arm them with a nuclear payload."

Netanyahu stressed that Iran is not governed by Rohani, but by its Supreme Leader Ayatollah Khamenei. During the interview he said Khamenei head a “messianic, apocalyptic, radical regime" and therefore cannot be trusted.