The United States is calling on North Korea to rejoin the Nuclear Nonproliferation Treaty (NPT).

State Department spokesman Philip Crowley told reporters Thursday that the U.S. wants to see North Korea take further steps toward denuclearization.

He added that the U.S. plans to start efforts on May third to strengthen the nonproliferation treaty.

Earlier on Wednesday, in an article contributed to British daily “The Guardian,” Secretary of State Hillary Clinton said the United States is pursuing diplomatic efforts that create real consequences for states such as North Korea and Iran that defy the global nonproliferation regime.

North Korea withdrew from the NPT in 2003 after the then U.S. President George W. Bush labeled North Korea part of an “axis of evil.”