Nikki Haley, the U.S. ambassador to the United Nations, speaks to the media on March 8 in New York, flanked by Ambassador Koro Bessho, left, of Japan and Ambassador Cho Tae-yul of South Korea. (Eduardo Munoz Alvarez/AFP/Getty Images)

The United States rebuffed a proposal from China to “apply the brakes” to an escalating standoff with North Korea, saying “positive action” was required before it would engage with the “irresponsible” Kim Jong Un.

China was trying to mediate the rising tensions in East Asia, which pit its once-close ally North Korea against the United States and its evolving foreign policy under President Trump.

The North Koreans have “given us enough reason to think how irresponsible they are,” with little cause to believe that “we’re dealing with a rational person on this,” said Nikki Haley, the U.S. ambassador to the United Nations. Speaking to reporters Wednesday after an emergency U.N. Security Council meeting on North Korea, she added, “We have to see some sort of positive action by North Korea before we can take them seriously.”

There has been no official Chinese reaction to the snub.

China’s foreign minister, Wang Yi, urged North Korea earlier Wednesday to suspend missile tests in return for a suspension of U.S.-South Korea military exercises.

(Jason Aldag/The Washington Post)

Wang compared the United States and North Korea to two “accelerating trains coming toward each other with neither side willing to give way.”

He cast China as a railway worker or signalman trying to keep things on track, though U.S. critics argue that China is not a neutral signalman but Pyongyang’s patron.

A “suspension for suspension” deal, Wang argued, was key to preserving peace and getting back to the negotiating table.

What he proposed has been pitched before by North Korea and rejected by the United States. But with the Trump administration still fleshing out its foreign policy, some experts wondered whether the proposal had a shot.

It did not. Flanked by colleagues from Japan and South Korea, Haley said the administration is still deciding how to proceed. “I can tell you we’re not ruling anything out and we’re considering every option that’s on the table,” she said. 

They may need to move quickly. The early months of the Trump presidency have coincided with a surge of tension in Asia, threatening to precipitate a conflict that could draw in North Korea, China, South Korea and Japan.

In February, North Korea tested a solid-fuel rocket that it claimed was part of an intercontinental ballistic missile capable of hitting the United States.

Soon after, the country was linked to the killing in Malaysia of Kim Jong Un’s half brother, Kim Jong Nam, who had been under Chinese protection.

Then, on Monday, as U.S.-South Korean military exercises got underway and China opened its not-to-be-interrupted National People’s Congress, Pyongyang pulled the trigger again, launching four missiles that landed in waters off the Japanese coast.

The same night, the United States and South Korea pushed ahead with the long-planned deployment of an antimissile system, called Terminal High Altitude Area Defense (THAAD), that China strongly opposes. 

At the center of this complex and evolving conflict are questions about China’s ties to North Korea and the U.S. role in East Asian affairs, particularly under Trump.

China and North Korea see the presence of U.S. troops in South Korea as the root of regional conflict, and they want the United States to acknowledge or address this by canceling or adapting joint military exercises.

The United States and South Korea are focused on the North Korean threat. They say THAAD is a defensive system designed to shoot down North Korean missiles.

But China believes that THAAD could be used to spy on Chinese airspace and sees the system as evidence of U.S. military “meddling” in Asian affairs. After the deployment on Monday, China’s Foreign Ministry warned of unspecified “consequences” for the United States and South Korea and has continued to target South Korean business interests. 

The United States, meanwhile, has pushed for China to play a more assertive role in reining in North Korea.

With Chinese leaders outraged over THAAD, the challenge for the Trump administration will be getting them to abandon their patronage of North Korea.

Jin Xin contributed to this report.