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

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

"We have an obligation," Kerry told reporters in Tokyo after he and Defense Secretary Chuck Hagel met with the foreign and defense ministers of Japan. "It would be diplomatic malpractice of the worst order not to examine every possibility of whether or not you can achieve that before you ask people to take military action and do what you have to do to prevent it."

In his first public comments since Israeli Prime Minister Benjamin Netanyahu warned the United States and the West not to trust Iran in an impassioned address to the United Nations, Kerry said the U.S. would not take any Iranian offer at face value and said Iran would have to prove it is not trying to develop a nuclear weapon.

"You have to exhaust the remedies before you ratchet up to a next tier of remedies that may have more dramatic consequences," he said.

Kerry, however, stressed that Rouhani's apparent overtures would be looked at with an extremely critical eye.

"There is nothing here that is going to be taken at face value and we have made that clear," he said. "It is not words that will make a difference, it's actions, and the actions clearly are going to have to be sufficient that the world will understand that not only will they not be on the road to get a weapon but there is no ability to suddenly break out and achieve that."

"I assure Netanyahu and all the Zionists that nothing that we do is going to be based on trust," Kerry said. "It's going to be based on a series of steps that guarantee to all of us that we have certainty about what is happening."

Senior diplomats from the five permanent members of the U.N. Security Council and Germany are preparing now to meet with top Iranian officials in mid-October in Geneva in the latest round of thus-far unsuccessful talks to get Iran to prove its nuclear program is peaceful.