US Secretary of State Mike Pompeo faced a testy exchange with Chinese officials in Beijing on Monday, on the final leg of an East Asian trip focused on the North Korean nuclear issue.
Pompeo traveled to China from South Korea where he said he and North Korean leader Kim Jong Un "refined options for the location and date" for the "upcoming second summit" between Kim and President Donald Trump.
Kim expressed enthusiasm for the future of talks between the two nations.
Pompeo tweeted after the meeting, "Had a good trip to #Pyongyang to meet with Chairman Kim".
Stephen Biegun, new US nuclear envoy who was accompanying the secretary, said he offered on Sunday to meet his counterpart, Vice Foreign Minister Choe Son Hui, "as soon as possible" and they were in discussion over specific dates and location. The visit was the latest indication of deteriorating ties between the US and China, as the two sides tussle over everything from trade to Taiwan and the South China Sea.
Wang appealed to Pompeo to cease actions that Beijing sees as threatening its interests in order to avoid disrupting cooperation over North Korea and other issues.
North Korea on Monday said it was ready to allow an worldwide group of inspectors into the country's clandestine nuclear and missile sites, a move that could be seen as more evidence in Washington and Pyongyang's thawing relationship. He also said that Seoul must cooperate with the United States and other concerned nations on "dismantling the last Cold War on Earth".
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The two countries have, however, been in a standoff over what the term denuclearisation means.
The North Korean leader also invited inspectors to visit the Punggye-ri nuclear test facility, which Pyongyang took apart in May, "to confirm that it has been irreversibly dismantled", the State Department said.
After Mr Pompeo left North Korea in July, hailing progress, Mr Kim's government denounced him for making "gangster-like demands".
The challenge for the US remains stark: American administrations for decades have been stymied by North Korean intransigence on its nuclear program, and the rapport between Trump and Kim hasn't resolved that issue.
Analysts say Washington may now consider new options as China, Russia and South Korea seek to relax sanctions.
The Secretary of State added that South Korea has been an integral part of negotiations aimed at denuclearising North Korea, and he passed along Mr Trump's gratitude for the country's efforts.
The US diplomat also met with Xi during a similar trip to the region in June.
"There is room for further reductions and I expect another one percentage point cut by the year-end", Xu added.