Japan and South Korea have held a joint military drill inside the Air Defence Identification Zone declared unilaterally by Beijing over a contested part of the East China Sea.
A spokesman for Japan's Maritime Self Defence Force said that two of its warships and a helicopter carried out a routine maritime rescue exercise with the South Korean navy in waters covered by the ADIZ.
"The drill had been planned for a long time, since before China's announcement. It was not organised in reaction," a Japanese navy spokesman told AFP.
The navy said its warships remained in international waters but deliberately failed to inform Beijing that the helicopter was flown through the zone, thus breaching China's demand that all aircraft passing through had to provide identification and a flight plan.
Tokyo and Seoul have refused to recognise the ADIZ, which was set up in November.
The area largely overlaps a Japanese air defence zone set up in 1969 and extends Beijing's aerial authority over a disputed group of islands known as Diaoyu in China and Senkaku in Japan. The archipelago has been the subject of ownership rows for decades.
The area also covers a submerged shelf that South Korea claims as part of its territory. Seoul has announced that it would expand its own air defence zone to include the reef.
The US and Australia condemned China's move as illegal and destabilising.
The Chinese foreign ministry rebuffed international criticism and repeated its hardline stance.
"China's activities here are just, reasonable and comply with international practices," spokesman Hong Lei said.