By David Brunnstrom

WASHINGTON (Reuters) - The top U.S. diplomat for East Asia said on Tuesday he was confident the United Nations would make "significant" progress on tougher sanctions against North Korea over its nuclear program, despite apparent differences with China over targeting coal exports.

China appears to have pushed back on a bid by Washington to close a U.N. loophole that allows North Korea to export coal for "livelihood purposes," saying the well-being of North Koreans is a priority in negotiations on possible new sanctions.

Asked about China's stance in the U.N. Security Council on a response to North Korea's fifth and largest nuclear test on Sept. 9, Daniel Russel, the U.S. assistant secretary of state for East Asia, told reporters:"It’s very important to get progress; We are determined to get progress.

"When it comes to negotiations, and particularly on negotiations in the first instance with China in New York at the U.N. Security Council, but more broadly with the 15 members, I will invoke the wisdom of that great sage Yogi Berra, who said, 'it ain't over 'til it’s over.'"

"We won’t know how far we will get ... but we know there will be a new Security Council resolution and I have great confidence that it will represent a significant ratcheting forward of the sanctions and the constraints on North Korea."

In March, the 15-member Security Council imposed tough new sanctions on North Korea following its fourth nuclear test in January.

That resolution bans the 193 U.N. member states from importing North Korean coal, iron and iron ore unless such transactions are for "livelihood purposes" and would not be generating revenue for Pyongyang's nuclear and missile programs.

U.S. Ambassador to the United Nations Samantha Power said on Sunday that some of the exemptions included in the March resolution, out of concern for the welfare of North Koreans, appeared to have been exploited.

A Chinese Foreign Ministry spokesman said on Tuesday China supported a further response by the Security Council but that it should focus directly on North Korea's nuclear program.

Russel said it was important to "enhance the enforcement" of the March sanctions. He said there was also more countries could do individually to "protect ourselves and the international infrastructure" from the North Korean threat.

Russel said the United States and its allies remained open to negotiations with North Korea - provided they focused on the nuclear issue.

(Reporting by David Brunnstrom; editing by Grant McCool)