Secretary of State John F. Kerry leaves after a press conference during U.S.-China economic talks in Beijing on Monday. (Lintao Zhang/Getty Images)

A U.S. spy plane was buzzed by Chinese jets Tuesday as it flew over the East China Sea, with one of the fighter planes approaching in an “unsafe” manner, the U.S. military said. It was the second such incident in three weeks.

China responded by accusing the United States of “hyping” the incident but said the real problem was U.S. surveillance planes flying too close to its territory.

Tensions between the two nations have risen in the past two years over China’s moves to assert sovereignty over the disputed waters and busy shipping lanes of the South China Sea. The United States accuses China of militarizing artificial islands built on reefs and rocks in waters that are also claimed by the Philippines, Vietnam, Malaysia, Taiwan and Brunei.

The U.S. Pacific Command said a U.S. Air Force RC-135 reconnaissance aircraft was intercepted Tuesday by two Chinese J-10 fighter planes while flying in international airspace.

China has laid claim to a number of islands in the South China Sea, building airbases on tiny spits of land while installing radar and missile batteries. Here's why. (Jason Aldag,Julie Vitkovskaya/The Washington Post / Satellite photos courtesy of CSIS)

“One of the intercepting Chinese jets had an unsafe excessive rate of closure on the RC-135 aircraft,” it said in a statement. “Initial assessment is that this seems to be a case of improper airmanship, as no other provocative or unsafe maneuvers occurred.”

Chinese Foreign Ministry spokesman Hong Lei insisted that his nation’s pilots always act responsibly and professionally.

“Judging by the report, the U.S. side is again deliberately hyping up the issue of the close surveillance of China by U.S. military aircraft,” he said Wednesday at a regular news conference.

“The root of relevant issues is that the frequent close reconnaissance on China’s coast by U.S. military aircraft severely threatens China’s air and maritime security,” he added. “China has the right to take defensive measures against it. We demand the U.S. side stop close reconnaissance activities and avoid the reoccurrence of such things.”

In May, the Pentagon said two Chinese fighter jets flew within 50 feet of a U.S. EP-3 aircraft over the South China Sea.

State news agency Xinhua responded to that incident by accusing the United States of flexing its military muscles on China’s doorstep and of “dangerous and irresponsible activities” that significantly increase the risk of military misjudgment.

The nationalist Global Times tabloid also reacted to the May incident by recalling a midair collision between a U.S. spy plane and a Chinese fighter jet over the South China Sea in 2001. That crash caused the death of a Chinese pilot and a forced landing by the U.S. plane.

“If the Pentagon continues its close-up offshore surveillance operations against China, as our military prowess increases, more interceptions can be expected. As a result, the odds of another collision will go up,” it wrote. “The simmering distrust between China and the U.S. will probably explode if there is another collision.”

On Sunday, Secretary of State John F. Kerry warned China not to consider declaring an “air defense identification zone” over the South China Sea, as it did over the East China Sea in 2013. He said such a move, which would assert control over air traffic in the region, would be a “provocative and destabilizing act.”

Gu Jinglu contributed to this report.