By Farnaz Fassihi

UNITED NATIONS -- The Security Council unanimously adopted a sanctions resolution aimed at penalizing North Korea for its nuclear-weapons program, cutting the country's annual revenue by about $800 million by means of a ceiling on the amount of coal it can export.

The resolution was supported by China, North Korea's closest ally, which said the security situation in the region had grown "dire" and was adopted on Wednesday.

For three months, diplomats, led by the U.S., negotiated over how to punish North Korea for repeated violations of United Nations sanctions banning the country from firing ballistic missiles and conducting nuclear tests. North Korea conductedits last and largest nuclear test on Sept. 9.

The last round of U.N. sanctions, in March, had failed to deter North Korea. Provisions meant to prevent the suffering of ordinary people by allowing certain types of trade were exploited by government officials to the benefit of the state, U.S. officials said.

In response, diplomats moved to curb North Korea's lucrative export of coal, its single largest source of revenue, estimated at more than $1.2 billion a year. The challenge for U.S. and its European allies on the Security Council was to persuade China, which remained the only country to import coal from North Korea and has been weary of drastic measures that would destabilize North Korea's economy, to go along.

Complicating the negotiations, diplomats said, Russia inserted itself into the bilateral negotiations between China and the U.S., delaying talks by raising its own concerns about the text of the resolution.

"Thewhole thing was difficult to overcome," said a Security Council diplomat. "The Chinese drove a very hard bargain...but something has changed in the Chinese calculus." The diplomat added that China appeared fed up and embarrassed by the behavior of North Korea.

China said it wanted to avoid a deterioration of security in the region, where it shares an 880-mile border with North Korea.

"The resolution reaffirms the need to safeguard the stability of the Korean peninsula," said China's ambassador to the U.N., Liu Jieyi. "The current situation in the Korean peninsula is sensitive, complex and dire."

Chinese experts say Beijing's decision to target a key source of Pyongyang's income was meant to admonish North Korean leader Kim Jong Un for his persistence in pursuing nuclear weapons.

"The political message encapsulated in the resolution is more important than the specific sanctions that it spells out," said Yang Xiyu, a Korean-affairs expert and senior fellow at the China Institute of International Studies, a Chinese Foreign Ministry-affiliated think tank.

The resolution was hailed by U.N. Secretary General Ban Ki-moon as the toughest and most comprehensive set of sanctions ever imposed by the Security Council and called on all member states to ensure they are fully implemented.

South Korea's intelligence agency estimates North Korea spent around $200 million on two nuclear tests and multiple missile launches this year.

North Korea hasn't reacted to the new sanctions, which will cap its coal exports at about $400 million or 7.5 million metric tons a year.

--Chun Han Wong contributed to this article.

Write to Farnaz Fassihi at farnaz.fassihi@wsj.com

(END) Dow Jones Newswires

November 30, 2016 19:21 ET (00:21 GMT)

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