Samantha Power said North Korea's "obsessive pursuit" of weapons of mass destruction poses a growing security threat on the Korean peninsula, in the Asia-Pacific region and beyond.
"The true measure ... Will be whether the rigor with which member states implement these sanctions matches or exceeds the rigor that (North Korea) will apply in attempting to evade these sanctions," she said, addressing a forum on the implementation of sanctions under a Security Council resolution.
The sanctions also crack down North Korea's trade in coal, iron ore and rare minerals.
Despite the sanctions, North Korean leader Kim Jong Un last week touted the successful launch of a powerful new midrange missile that was propelled more than 1,400 kilometers (870 miles), putting much of Asia and the Pacific within range.
Also Read
The test indicated technological advances in the North's missile capabilities and was quickly condemned by Washington, Tokyo and Seoul as a "provocation" and a violation of United Nations resolutions.
South Korean Ambassador Oh Joon said that after four nuclear tests over 10 years, the debate over whether North Korea uses nuclear development as a bargaining chip to obtain economic cooperation or is in fact sincerely pursuing nuclear weapons has been settled.
"They've never had the genuine willingness to give up nuclear weapons through negotiations," he said. As a result, the international community has to make it tougher on North Korea, achieving its goals through sanctions, Oh said.