The 15-member Security Council had until 2000 GMT to decide on the US request, submitted last week. China's block came before a new closed-door council meeting on North Korea, also requested by Washington.

Under current UN rules, the block period lasts for six months. After that, another council member can extend the block for three more months and one day, before the proposal is permanently removed from the negotiating table.

It is possible that other countries will also block the US request by the 2000 GMT deadline, diplomats said, implying Russia might do so. Along with Beijing, Moscow has long held a line against increasing pressure on North Korea, even asking for relief from international sanctions for humanitarian reasons.

Last week, after Washington levied sanctions on five North Koreans linked to the country's ballistic missile program, the United States undertook a campaign within the Security Council to extend UN sanctions to those same five people.

The US Treasury Department said one of the North Koreans being sanctioned, Choe Myong Hyon, was based in Russia and had provided support to North Korea's Second Academy of Natural Sciences (SANS), which is already subject to sanctions.

Also targeted were four China-based North Korean representatives of SANS-subordinate organizations, the Treasury Department said: Sim Kwang Sok, Kim Song Hun, Kang Chol Hak, and Pyon Kwang Chol.

Washington has accused all five of ties to North Korea's weapons of mass destruction program.

The Chinese diplomatic mission to the UN did not immediately respond to a request for comment.