By Yara Bayoumy and Yeganeh Torbati
WASHINGTON, Sept 9 (Reuters) - North Korea's fifth nuclear test, its fourth on U.S. President Barack Obama's watch, leaves Washington yet again hoping that Beijing will crack down on Pyongyang.
Given China's longstanding fear of a North Korean collapse that could send thousands of refugees across their 870-mile (1,400 km) border, North Korea analysts said significantly tougher Chinese economic sanctions on North Korea are highly unlikely.
They said the best hope is that China might better enforce or tighten U.N. sanctions by eliminating a loophole that allows Chinese imports of North Korean coal, cutting the remittances of North Korean workers or limiting the work of Chinese factories that process North Korean textiles.
Mark Fitzpatrick, executive director of the IISS-Americas think tank, said it was worth seeing if China might increase pressure on the North in return for the United States and South Korea halting plans to deploy the Terminal High Altitude Area Defense (THAAD) anti-missile defense system designed to protect against North Korea's missile and nuclear threats.
China has said the THAAD system would destabilize the regional security balance without achieving anything to end North Korea's nuclear program.
"We can't assume that China is going to solve this for us," Fitzpatrick said. "If there was a prospect of a tradeoff I think China would consider it," Fitzpatrick said, referring to introducing THAAD as a bargaining chip.
North Korea conducted its fifth and biggest nuclear test on Friday and said it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat that rivals and the United Nations have been powerless to contain.
Under 32-year-old third-generation leader Kim Jong Un, North Korea has sped up development of its nuclear and missile programs, despite U.N. sanctions that were tightened in March and have further isolated the impoverished country.
There is little scientific evidence to verify that North Korea has perfected the science of creating a nuclear bomb small enough to fit on a ballistic missile, let alone to withstand the physics of atmospheric re-entry.
But analysts said it may be getting closer and its testing of ballistic missiles, including those designed to launch from submarines, accentuates the threat it poses to U.S. allies South Korea and Japan and ultimately the United States itself.
U.N. Security Council Resolution 2270, which was passed in March after the North's fourth nuclear test, provided a loophole allowing imports of North Korean coal if such transactions are solely for the North's "livelihood" and will not yield revenue for its nuclear, ballistic missile or other restricted programs.
Mira Rapp-Hooper of the Center for a New American Security think tank said the international community could crack down by limiting Chinese processing of North Korean textiles and by restricting the earnings that North Koreans abroad send home, perhaps by curtailing the number of visas given to such workers.
"I think the fifth test will be an occasion where we can close some of the loopholes of the previous sanctions," said Robert Manning, a fellow at the Atlantic Council's Brent Scowcroft Center on International Security.
He noted that 90 percent of North Korea's trade is with China.
President Barack Obama in March signed into law sweeping new "secondary sanctions" allowing the United States to go after foreign firms that do business with North Korea by effectively barring them from the global financial system, analysts said it was unclear if he would do so before his term ends in January.
(Reporting By David Alexander, Yara Bayoumy, Arshad Mohammed and Yeganeh Torbati; Writing by Arshad Mohammed; Editing by Grant McCool and Tom Brown)
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