BEIJING -- China denied U.S. accusations that its jets conducted an unprofessional intercept of an American radiation-sniffing surveillance plane in the East China Sea and urged Washington to stop such activities.

The two Chinese SU-30 jets on Wednesday approached a WC-135 Constant Phoenix aircraft -- a modified Boeing C-135 -- conducting a routine mission in international airspace in accordance with international law, Pacific Air Forces spokeswoman Lt. Col. Lori Hodge said in a statement.

The WC-135 crew characterized the intercept as unprofessional "due to the manoeuvrs by the Chinese pilot, as well as the speeds and proximity of both aircraft," Hodge said.

She declined to provide further details and said the issue would be addressed with China through "appropriate diplomatic and military channels."

"We would rather discuss it privately with China," Hodge said in an email to The Associated Press. "This will allow us to continue building confidence with our Chinese counterparts on expected manoeuvring to avoid mishaps."

In China, Defence Ministry spokesman Wu Qian said the American plane was conducting surveillance over the Yellow Sea -- the northern part of the East China Sea -- and that the Chinese jets moved to identify and verify the plane "in accordance with laws and regulations."

In a statement on the ministry's website, Wu said the operation was "professional and safe." Wu blamed surveillance by U.S. planes and vessels as the "root cause of military security problems concerning sea and air" between the two countries and urged the U.S. to stop such activities.

China declared an air defence identification zone over a large section of the East China Sea in 2013, a move the U.S. called illegitimate and has refused to recognize.

China has demanded foreign aircraft operating within the zone declare their intentions and follow Chinese instructions. Hodge declined to say whether Wednesday's incident was within the self-declared Chinese zone.

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

Unexpected and unsafe intercepts involving U.S. and Chinese military aircraft have occurred occasionally over the South China Sea, which China claims virtually in its entirety. Although China says it respects freedom of navigation in the strategically vital area, it objects to U.S. military activities, especially the collection of signals intelligence by U.S. craft operating near the coast of its southern island province of Hainan, home to several military installations.

In recent years, the sides have signed a pair of agreements aimed at preventing such encounters from sparking an international crisis, as happened in April 2001 when 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 China's detention of the 24 U.S. crew members for 10 days.

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Associated Press writer Christopher Bodeen contributed to this report.