In the wake of North Korea's nuclear weapons test last month and its long-range missile test in early February, the U.S. and China have agreed on a draft U.N. resolution imposing new sanctions on Pyongyang. North Korea is already under a raft of international sanctions, but the new proposal would tighten them and impose new bans.

U.S. Ambassador to the United Nations Samantha Power tells NPR's Robert Siegel that the resolution, submitted Thursday to the Security Council, "is nearly unprecedented in many respects and is the toughest sanctions resolution that has been put forward in more than two decades." A vote is expected in the coming days.

The proposed resolution includes imposing mandatory inspection of all cargo leaving or entering North Korea and a ban on the sale of all small arms and other conventional weapons to North Korea. It also limits or bans certain exports and prohibits supplying North Korea with aviation fuel, including rocket fuel.

China is North Korea's most important ally and historically has opposed punishing the regime. But it has given support to past Security Council resolutions imposing sanctions on Pyongyang.

Power notes that the relationship between the two countries has grown more complicated recently.

"Even over the last couple years, we've seen Chinese acute frustration with Kim Jong-un, the leader of North Korea," she says. "And we have seen China not always being able to influence the regime in the way that they have sought. And I think that's been a source of frustration. I think China's genuinely worried about the threat that Kim Jong-un and his nuclear weapons program poses to the peninsula and to international peace and security."

The U.S. had already been in negotiations with China over North Korea's nuclear capabilities when North Korea conducted a nuclear test in January, which it claimed to be a hydrogen bomb. Power says this was a turning point.

"After the nuclear test, when we had already started the negotiations on this resolution," she says, "China sent its nonproliferation envoy to Pyongyang in order to urge that they do nothing else, particularly while these negotiations were under way."

After the envoy left, she says, "The North Korean regime fired the equivalent of a ballistic missile – a space launch – and I think that was something that ... certainly on the basis of how our negotiations picked up speed at that point, I think that was a source of great irritation for China."

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