SEOUL, South Korea (AP) — Top nuclear envoys from South Korea, the United States and Japan pledged Tuesday to implement new sanctions against North Korea, including capping the country's coal exports to choke off cash flowing into its nuclear weapons program.

After meeting in Seoul, the envoys said the countries agreed to maintain an around-the-clock information sharing system to monitor how sanctions are working.

Responding to North Korea's nuclear and missile tests this year, the United Nations Security Council recently voted to tighten sanctions by limiting the country's coal exports, one of its major sources of foreign currency.

The effectiveness of the sanctions is dependent on the commitment of China, North Korea's most important ally and its biggest buyer of coal. Joseph Yun, U.S. special representative for North Korea policy, said China's announcement over the weekend that it would freeze imports of North Korean coal until the end of the year was an encouraging sign.

"We look forward to more cooperation from the Chinese authorities, and with their full cooperation, I do believe the intended effect of cutting revenues that go to North Korea will be done," Yun said.

After the toughening of the U.N. measures, South Korea and Japan also announced new, unilateral sanctions against North Korea.

South Korea banned its citizens from engaging in financial dealings with dozens of high-profile North Korean officials and entities. The North Korean officials include two of leader Kim Jong Un's closest associates, Choe Ryong Hae and Hwang Pyong So, and the blacklisted entities include those involved in coal exports and the dispatch of laborers abroad.

Japan said it will expand its blacklist of individuals, groups and vessels suspected of ties to North Korean weapons programs. It also plans to more strictly block visits by North Korean officials and penalize related groups, including ones in China.