Japan has lodged a protest with China over its “continued deployment” of a drilling ship at a gas field in the East China Sea, which has been the subject of a territorial dispute between the two sides.

The protest, the second in a week, was conveyed to Beijing through diplomatic channels after Tokyo said on Saturday that a Chinese drillship had moved to a location a few kilometres northeast of the sea in January.

“It's extremely regrettable that China has continued its unilateral development activity,” Chief Cabinet Secretary Yoshihide Suga said at a press briefing.

Japan accuses China of trying to syphon off resources from beneath the Japanese side of the line, a charge China denies.

Tokyo and Beijing have for several years been locked in a territorial row over a small group of uninhabited islands in the East China Sea, known respectively as the Senkaku or Diaoyu islands. 

China maintains that it has indisputable sovereignty over the islands. The Japanese government, however, regards them as a part of its territory.

The two sides agreed on joint gas development in the area in 2008 but negotiations were suspended in 2010. Ties, however, deteriorated after Tokyo nationalized part of the resource-rich islands in 2012.

Beijing claims sovereignty over almost all of the disputed territory in the South and East China seas. Vietnam, the Philippines, Malaysia, Brunei and Taiwan also have competing claims.

The South China Sea is believed to sit atop vast oil and gas reserves. The territorial dispute usually draws in trans-regional countries, particularly the US.

Beijing accuses Washington of interfering in the regional issues and deliberately stirring up tensions in the strategic waters of the South China Sea.

US activity near the resource-rich international waters off China's coast has long been a source of tension between the two sides.

China accuses the US of carrying out reconnaissance flights over Chinese coastal waters and frequently calls on the US to halt patrols in the area.