Two Chinese fighter jets this week buzzed a U.S. spy plane that sniffs out nuclear radiation as it flew over the seas between China and North Korea, underlining Beijing’s discomfort with American surveillance and differences in the countries’ approach to the threat posed by the regime in Pyongyang.

A Defense Department official said the Chinese aircraft got within 100 feet or so of the U.S. plane.

The incident, reported by the U.S. Air Force on Friday, comes as the two nations struggle to overcome discord over how to confront the nuclear and missile programs of North Korea, which depends on China as its main economic lifeline.

The American plane, a WC-135 Constant Phoenix, collects samples from the air to detect nuclear explosions. It is used worldwide to monitor compliance with international obligations. It flies about 150 missions a year, the defense official said.

The Air Force said the plane was on a routine mission in international airspace in the East China Sea, a broad expanse of ocean. But China’s Ministry of National Defense said the incident occurred in the Yellow Sea, the northern part of the East China Sea that lies between China and the Korean Peninsula.

An American official told CNN that the plane has been regularly deployed in northeast Asia to gather evidence of possible nuclear tests by North Korea.

The Chinese military has become increasingly unhappy with U.S. surveillance in the East China and South China seas. Intercepts are not uncommon in both areas. But the latest incident appeared to have been an unusually tight encounter.

Two Chinese Su-30 fighters flew up close to an American WC-135 on Wednesday, the Air Force said in a statement. The American aircrew described the intercept as “unprofessional,” the statement said, based on the Chinese pilots’ maneuvers and the speeds and proximity involved.

“The issue is being addressed with China through appropriate diplomatic and military channels,” said Lt. Col. Lori Hodge, an Air Force spokeswoman. A U.S. military investigation into the intercept is underway, she added. 

China’s Defense Ministry said that the Air Force statement was not factually correct and that the Chinese planes had “recognized and checked on” the American plane as it conducted surveillance “in China’s Yellow Sea.”

“The relevant behavior was professional and safe,” China said in a statement published on its social-media account.

“The root reason for military security issues between the air force and navy of China and the United States is that U.S. planes and ships frequently conduct surveillance near China’s territory,” the Defense Ministry said. “We hope the U.S. side can pause relevant surveillance activities, to prevent such things from happening.”

Chinese Foreign Ministry spokeswoman Hua Chunying asked the United States to respect China’s national security concerns. 

“As we said before, for a long time, the surveillance activities of U.S. military planes and ships near China’s territory are very likely to lead to misunderstanding, miscalculation and accidents on the sea or in the air,” she told reporters at a news conference.

Whether the close encounter was meant to send a signal to the Americans or was merely the action of overly zealous pilots is not clear, experts said.

The American official told CNN that the Chinese jets came within 150 feet of the U.S. plane, with one of them flying upside down directly above it. 

“U.S. military aircraft routinely transit international airspace throughout the Pacific, including the East China Sea,” Hodge said. “This flight was no exception.”

China is also deeply unhappy about the deployment in South Korea of a U.S. missile defense system, which is meant to protect the country against attack from North Korea but which Beijing fears will also be used to spy on its territory.

Du Xiaojun, an expert in East Asian security at Guangxi University for Nationalities, said the incident Wednesday highlighted the “security dilemma” between China and the United States. 

“The United States, Japan and South Korea always say their moves are targeting North Korea, but China has many reasons not to believe so,” Du said. “It’s hard for China to tell whether the surveillance was targeting North Korea or China’s military deployment.” He compared the mistrust to American concerns about China’s island-building in the South China Sea, which Beijing insists is primarily for civilian use — despite having built military airstrips and missile shelters there.

“Both sides suffer from misunderstandings and lack mutual trust,” Du said.

President Trump has praised Chinese President Xi Jinping for helping the United States pressure North Korea to dismantle its nuclear and missile programs, and China says it is enforcing United Nations sanctions and has suspended coal imports.

However, experts say Beijing is unwilling to push Pyongyang too far and does not want to see the regime fall. It is urging dialogue between the two sides. 

This latest incident, combined with the furor over the deployment of the Terminal High Altitude Area Defense (THAAD) system in South Korea, is a reminder of how even defensive actions by the United States relating to North Korea can spook China.

More than that, it underlines the tensions over U.S. military surveillance in the region.

In December, a Chinese naval ship seized an underwater American drone in international waters in the South China Sea. It was returned the following week. 

In February, a Chinese military plane crossed near the nose of a Navy P-3C Orion anti-submarine and surveillance plane above the South China Sea, flying within about 1,000 feet in an encounter the Pentagon described as unsafe but probably unintentional.

In April 2001, a Chinese jet fighter collided with a U.S. surveillance plane over the South China Sea, leading to the death of the Chinese pilot and the detention of the 24 U.S. crew members in China for 10 days. Since then, the two countries have established closer contacts to prevent similar incidents from sparking an international crisis. 

Luna Lin and Shirley Feng contributed to this report.