GENEVA » Ahead of a round of global nuclear talks, five major powers labeled North Korea and Iran as “serious challenges” to the world’s nuclear security today, citing their repeated defiance of U.N. sanctions.

Senior diplomats with the U.N. Security Council’s five permanent members singled out North Korea’s nuclear test in February and Iran’s “continued pursuit of certain nuclear activities” as among the biggest threats to the 1970 Nuclear Non-Proliferation Treaty, the world’s most important pact on preventing the spread of nuclear arms.

A joint statement by Britain, China, France, Russia and the United States also called for establishing the Middle East as a zone free of nuclear weapons.

The statement preceded two weeks of talks in Geneva to prepare for a broad review of the NPT, which has been signed by 190 nations and is credited with preventing the spread of nuclear weapons to dozens of nations in the 43 years since it entered into force. Iran is a member, but North Korea — along with India, Pakistan and Israel — are not.

The treaty commits nations without nuclear weapons to refrain from acquiring them. Those with them commit to take steps toward their elimination. All who sign agree that everyone has a right to develop peaceful nuclear energy.

Nations meet every five years to review the treaty and try to find new approaches to old problems. The next such review will be in 2015, and preparatory meetings like the one in Geneva draw on increasing cooperation between the Security Council’s permanent members, said Rose Gottemoeller, acting U.S. undersecretary of state for arms control and international security.

“Between now and the review conference you will see increasing payoff, I think,” she said.

Iran says it’s enriching uranium only for nuclear power and other non-military applications, and insists it has no interest in nuclear weapons. But the United States, Israel and their allies fear Iran may create weapons-level uranium that can be used in an atomic bomb, based on Tehran’s nuclear secrecy and suspicions they share with the International Atomic Energy Agency that Iran may have worked secretly on nuclear arms.

The U.N. Security Council has demanded a stop to both that effort and all enrichment in a series of resolutions since 2006.

North Korea has been sustained by Chinese food and fuel, and by growing Chinese trade and investment. But after North Korean ruler Kim Jong Un’s February nuclear test, China signed onto the toughest U.N. sanctions yet on Pyongyang.

At a news conference today, however, Sen Pang, director-general of the Chinese Foreign Ministry’s Arms Control and Disarmament Department, cautioned against a “vicious cycle” of confrontation with North Korea that could lead to war.

“The key issue here is the lack of security and the lack of mutual trust,” he said. “Any one side’s increase of military force will cause insecurity on the other side, and will cause a corresponding response.”

North Korea had committed itself to scrapping its nuclear program, including a presumed small stockpile of weapons, in a 2005 agreement with China, Japan, Russia, South Korea and the United States. In return, it was promised aid and diplomatic incentives. But Pyongyang walked out of talks in 2009 and later conducted more nuclear tests.

Recently, North Korea has warned that its weapons are on standby pointed at its enemies, but it has not divulged plans to carry out another nuclear test or fire a missile.

“We think that all sides should demonstrate restraint in this very tense situation,” said Grigory Berdennikov, ambassador-at-large for Russia’s Foreign Ministry, which hosted the joint news conference.