Indonesia has mobilised its military after Chinese fishing boats flanked by coastguard warships forced their way into its territory.

Six Indonesian vessels have been deployed to the Natuna archipelago in the South China Sea where the Chinese incursions began late last year. Four more ships are expected to follow.

"We are not only deploying ships, but also fighter jets. We are on full alert," Indonesian military spokesperson major general Sisriadi is quoted as saying in media reports.

Chinese foreign ministry spokesperson Geng Shuang said last week the ship's activities were "routine".

"The China Coast Guard were performing their duty by carrying out routine patrols to maintain maritime order and protect our people's legitimate rights and interests in the relevant waters."