"It seems China was attempting to spy on Korea and the U.S. in preparation for resumption of joint military drills," a government source here said. The Defense Ministry summoned three military attachés of the Chinese Embassy in Seoul to lodge a protest.

The aircraft came within 33 km of Korean territorial waters. Several Chinese military aircraft have buzzed the Korean ADIZ in recent months, but mostly in areas where the two countries' zones overlap and never so close to Korean territorial waters.

A Chinese military reconnaissance plane flew into Korea's air defense identification zone without warning on Tuesday. It flew to an area about 55 km northwest of Ulleung Island before turning back, according to the Joint Chiefs of Staff here.

The military aircraft entered the zone from southwest of Ieo Island around 9:34 a.m., the JCS said.

It was an Y-9 intelligence gathering aircraft that is capable of monitoring electronic communications. Afterwards, it flew over the Korea Strait, zigzagging across the Korean and Japanese air defense identification zones.

Around 11 a.m., it turned north from waters southeast of Busan and flew back into the Korean zone about 74 km from the coastline and kept going north until it reached waters some 55 km northwest of Ulleung Island.

Korea scrambled around a dozen fighter planes to intercept it. When it came too close to Ulleung Island, the Korean military warned the Chinese aircraft and its home base to stop provocations.

The Korean fighter jets flew just a few hundred meters from the Chinese military aircraft to identify it with the naked eye. The Chinese military claimed it was a routine training flight. The aircraft turned back by the same route around 11:34 a.m. It finally left the zone around 2 p.m.

In the meantime, two Chinese fighter jets were on standby southwest of Ieo Island, according to the JCS. "We speculate that the Chinese military aircraft buzzed the KADIZ to gather intelligence that can have effects on our military operations," a JCS spokesman said.

Air defense identification zones are not territorial airspace but require incoming planes to identify themselves to the country that claims them.

