A drastic improvement in relations between China and North Korea is behind U.S. President Donald Trump's sudden cancellation Thursday of his talks with Pyongyang's leader Kim Jong Un, diplomats in Beijing suspect.

North Korea has succeeded in obtaining support of the Asian power since the spring and started to take a tougher stance against the United States in the run-up to the Kim-Trump summit that was supposed to take place on June 12 in Singapore.

Trump's reaction, however, has gone far beyond Pyongyang's expectations, forcing Kim to reconsider his strategy to achieve North Korea's cherished goal of obtaining meaningful security guarantees from the United States, the diplomats said.

Recently, Sino-North Korea ties have greatly improved. Kim traveled to Beijing in March to talk with Chinese President Xi Jinping, and earlier this month the two leaders met again over two days in the northeastern Chinese city of Dalian.

Kim's visit to China in March was his first foreign trip since becoming North Korea's supreme leader following the death of his father Kim Jong Il in 2011. Since then, several senior officials of the two countries have made reciprocal visits.

Many foreign affairs experts say Beijing has already agreed to back Pyongyang up in promoting negotiations with the United States.

U.S. diplomats in Beijing have been carefully monitoring interactions between China and North Korea, a source familiar with the matter told Kyodo News.

When speculation grew Thursday of a North Korea high-ranking official visiting Beijing, the diplomats struggled to obtain details, the source said, with some even calling up journalists for tidbits of information.

Ahead of Trump-Kim talks, the United States has been "really interested in what China and North Korea have discussed, whether China and North have made deals, and how improved China-North relations would affect the U.S.-North summit," the source added.

Trump has expressed frustration over Kim's approach to Xi, saying he detected a shift in North Korea's tone in the wake of their May 7-8 summit in China.

"There was a somewhat different attitude after that meeting, and I'm a little surprised," Trump said in a meeting with South Korean President Moon Jae In at the White House earlier this week, adding, "I don't like that...I can't say I'm happy about it."

(Kim Jong Un, left, and Xi Jinping)
[KCNA/Kyodo]

Chinese Foreign Ministry spokesman Lu Kang told reporters on Friday, "We have no ulterior intention."

But Tong Zhao, a fellow at the Carnegie-Tsinghua Center for Global Policy in Beijing, said China and North Korea have "special" ties as communist countries, adding it is likely for them to make behind-the-scenes contacts before the U.S.-North Korea summit.

A diplomatic source from an Asian nation said China may have agreed to assist North Korea to finalize negotiations with the United States, while encouraging Kim to play hardball with Trump.

Since last week, North Korea had threatened to cancel the summit, lambasting Washington's pressure on Pyongyang to abandon its nuclear weapons before receiving reciprocal benefits.

U.S. Secretary of State Mike Pompeo also told the Senate Foreign Relations Committee on Thursday, "Over the past many days...we have received no response to our inquiries" from North Korea.

Nevertheless, Trump, being the personification of alpha-male one-upmanship, has been outplaying Kim, the diplomatic source said. In addition to the abrupt cancellation of the summit, the businessman-turned-president has started to warn that the United States may resort to military force.

(Mike Pompeo, left, and Kim Jong Un)
[KCNA-UPI/Kyodo]

Tetsuo Kotani, an associate professor at Japan's Meikai University, said in a TV program that North Korea "has taken the wrong attitude."

Kim is believed to be keen to ask the United States to accept the continuation of North Korea's hereditary authoritarianism, in return for Pyongyang vowing to denuclearize in a "phased" and "synchronized" manner.

Hours after Trump's announcement, Kim Kye Gwan, a vice foreign minister, swiftly released a statement urging the United States to reconsider its summit cancellation.

"We would like to make known to the U.S. side once again that we have the intent to sit with the U.S. side to solve problem regardless of ways at any time," the senior official said in the statement carried by Pyongyang's state-run Korean Central News Agency.

"We remain unchanged in our goal and will to do everything we could for peace and stability of the Korean Peninsula and humankind," the official said in the statement, adding, "We, broad-minded and open all the time, have the willingness to offer the U.S. side time and opportunity."