BEIJING (Reuters) - China’s Commerce Ministry said it will put a temporary ban on imports of North Korean coal as part of a U.N. Security Council resolution meant to deter Pyongyang from pursuing its nuclear weapons program.

The 15-member Security Council late last month put new sanctions on North Korea aimed at cutting its annual export revenue by a quarter after it carried out its fifth and largest nuclear test so far in September.

“To implement United Nations Security Council Resolution 2321, according to the People’s Republic of China’s Foreign Trade Law, imports of coal produced in North Korea temporarily will be stopped,” the ministry said in a short statement on its website on Saturday.

China, believed to be the only country buying North Korean coal, would slash its imports by some $700 million compared with 2015 sales under the new sanctions, according to diplomats.

Despite its recent anger at Pyongyang’s nuclear and missile tests, Beijing remains North Korea’s most important economic and diplomatic backer and it frequently comes under suspicion for not properly enforcing sanctions.

It said the ban will be in effect until the end of the year, though coal shipped before Dec. 11 that was yet arrive at Chinese customs would be exempt.

Over the first 10 months of this year, China imported 18.6 million tonnes of coal from North Korea, up almost 13 percent from a year ago.

Coal is one of North Korea’s only sources of hard currency and its largest single export item.

North Korea has said any sanctions against its missile or nuclear programmes are a violation of its sovereignty and right to self-defense.

It is technically still at war with democratic South Korea because their 1950-53 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and the South’s main ally, the United States.

Pyongyang has been under U.N. sanctions since 2006 over its nuclear and ballistic missile tests. It conducted its latest nuclear test on Sept. 9.