Washington, Apr 30 (DPA): The nuclear activities of Iran and North Korea pose a threat to a key nuclear treaty designed to prevent the spread of atomic weapons, a top US official warned Thursday.

North Korea's withdrawal from the nuclear Non-Proliferation Treaty (NPT) in 2003 and Iran's intent to develop nuclear weapons by masking it as an energy programme undermine the goals of the treaty, said Ellen Tauscher, under secretary of state for arms control.

"The nuclear non-proliferation regime is under great stress and is fraying at the seams," Tauscher said ahead of UN conference that begins in New York next week for the five-year review of the NPT.

North Korea conducted tests of two nuclear bombs after withdrawing from the NPT. Pyongyang agreed through years of six-nation negotiations to abandon its nuclear programme, but the deal has not been implemented.

Following the test the US and its partners enacted UN Security Council sanctions against the Stalinist state.

President Barack Obama's administration is working with key European allies to persuade the Security Council to introduce another round of sanctions on Iran, accusing the Islamic republic of refusing to meet its international obligations to come clean on its nuclear activities.

Tauscher warned that North Korea's and Iran's pursuit of atomic weapons threaten to unravel the NPT and international efforts to prevent the spread of nuclear armaments.

"This cynical path to a nuclear weapon cannot be allowed to serve as a model for others, otherwise it strikes at the very core bargain of the treaty," she told a gathering at the Center for American Progress, a think tank in Washington.

Iran is expected to be a major focus at the UN conference that begins on Monday and runs until May 28. US Secretary of State Hillary Clinton is heading the American delegation - the highest US representative to attend in 10 years.

Clinton will likely be meeting with her counterparts on the sidelines to win support for additional sanctions on Iran. Iran flatly denies it is pursuing weapons and maintain the programme is solely for producing energy - which is permitted under the NPT.

Iranian President Mahmoud Ahmadinejad has requested a visa from the US government so he can attend the event and make his country's case.

Arab governments participating will likely urge the world to act against Israel for its unacknowledged possession of nuclear weapons. Tauscher said the United States, Israel's closest ally, stands by a 1995 Security Council resolution calling for a Middle East free of nuclear weapons.

She said the best way to achieve that goal is to reach a "lasting and just peace in the Middle East".