The nuclear programmes of Iran and North Korea threaten the credibility of the global treaty aimed at avoiding the proliferation of atomic weapons, a high-level US official warned Monday.

North Korea's third nuclear test earlier this year and Iran's "nuclear enrichment programme which far exceeds any conceivable civilian use" constitute "the real challenges to the non-proliferation treaty," said Thomas Countryman, US assistant secretary of international security and nonproliferation.

Speaking to reporters on the sidelines of a preparatory meeting for the 2015 review conference of the Non-Proliferation Treaty (NPT), Countryman expressed particular concern about the consequences if Iran, which remains a signatory to the treaty, were to obtain nuclear weapons.

The West believes Iran is working towards developing a nuclear weapons capability, although the country itself insists its atomic work is exclusively for energy and medical purposes.

"I do consider that the possibility of Iran acquiring such a weapon would be a genuine tipping point and would cause more damage to the treaty than anything else that has occurred in its history," Countryman said.

If that happened, he warned, the NPT treaty which since 1970 has set the global agenda for keeping countries from getting the bomb would be rendered irrelevant.

"The possession of such weapons by Iran constitutes a threat to the entire region and an impetus for greater lateral proliferation of nuclear weapons than we have ever seen," he said.

Diplomatic efforts have been underway for years in which the five permanent members of the UN Security Council -- all nuclear-armed powers -- plus Germany have tried, so far in vain, to persuade Iran to cut back on sensitive aspects of its nuclear work.

Countryman also expressed deep concern over North Korea, which withdrew from the NPT in 2003. The Asian nation has since launched three nuclear tests and has in recent months been locked in increasingly hostile exchanges with South Korea and the United States, including threats of nuclear war and precision missile strikes.

"The case of North Korea makes clear that the treaty can be abused by those who say they have withdrawn," he said. He pointed out that the country had acquired nuclear weapons technologies when it was still a member of the treaty.

"There are other states today that are members of the treaty, but are acquiring technology that appears determined to be used for nuclear weapons," he said, insisting: "There needs to be consequences for such actions."

For both Iran and North Korea, Countryman stressed the need for "concerted, international diplomatic action."

"It is important that the world responds calmly but deliberately without changing our emphasis," he said.