The UN Security Council agreed Tuesday to a request from China to lift sanctions imposed on four ships that had been blacklisted over North Korea's weapons program, diplomats said.

They were affected when the UN Security Council on March 2 imposed tough sanctions on North Korea in response to its fourth nuclear test and rocket launch, widely seen as a disguised ballistic missile test.

Tuesday's move affects four of 31 ships linked to North Korean shipping firm Ocean Maritime Management, blacklisted for ties to Pyongyang's nuclear and weapons programs.

"The council has agreed to the request," a council diplomat told AFP. China gave assurances that the four vessels would not use North Korean crews.

The March 2 resolution was drafted by the United States but backed by China, North Korea's sole ally and main trading partner.

It required countries to inspect all cargo to and from North Korea, impose trade restrictions and bar vessels suspected of carrying illegal goods for North Korea from ports.

North Korea has been hit by five sets of UN sanctions since it first tested an atomic device in 2006.