TOKYO:  Japan plans to take "countermeasures" if China starts drilling in a disputed gas field in the East China Sea amid a simmering territorial row, press reports said Sunday.

Japan says aerial photographs show that China has recently transported what appears to be drilling equipment to the the gas field, known in Japan as Shirakaba and in China as Chunxiao.

Tokyo`s stance was confirmed when Prime Minister Naoto Kan met on Saturday with key cabinet members, including Foreign Minister Seiji Maehara, to discuss China`s activities in the area, the Kyodo news agency and the Nikkei daily said.

Possible countermeasures include a plan for Japan to conduct its own test drilling in the sea near the Chinese offshore facility, the reports said.

Nikkei said the Japanese Foreign Ministry was considering bringing a case against China before the International Tribunal for the Law of the Sea.

Maehara told reporters on Friday Japan would take "necessary actions" if it became clear that China was extracting gas from the field.

Beijing said on Friday its activities in the gas field were "completely reasonable and legal".

The field lies in an area where both countries` claimed exclusive economic zones overlap and has long been a bone of contention between the growing competitors for energy and mineral resources.

Japan plans to reinforce surveillance by naval P-3C patrol aircraft in the area around the gas field, Kyodo quoted government sources as saying.

The neighbours are entangled in their worst spat in years, stemming from the September 7 collision of a Chinese fishing trawler and two Japanese coastguard vessels near a disputed island chain in the East China Sea.

China has so far summoned Japan`s ambassador five times over the incident, cancelled a senior lawmaker`s Tokyo visit, and repeatedly demanded the boat`s captain be released from Japanese custody.

BDST: 1100 HRS, September 19, 2010