China Marine Surveillance (CMS) patrol ships had a busy year in 2012 due to the escalation of a territorial dispute between China and Japan over the Diaoyu Islands, according to CMS officials.

CMS sailors were not permitted to take turns going on holiday last year, with some sailors spending as long as four months at sea.

Only a few of the CMS's vessels are capable of reaching the Diaoyu Islands.

"Even if they are allowed to return, the crew members often receive new orders almost as soon as their vessel reaches port. Sometimes the orders come halfway through their journey back," said Wang Yun, captain of the Haijian 83, a CMS flagship patrol vessel.

Relations between Japan and China soured after the Japanese government said it would "purchase" part of the Diaoyu Islands in the East China Sea in 2012. China insists that the islands are part of its inherent territory.

Japan's move resulted in protests from the Chinese government and public. In September 2012, Chinese marine surveillance ships began regular patrols in waters around the Diaoyu Islands.

On Monday, the State Oceanic Administration (SOA) confirmed that a fleet of four Chinese marine surveillance ships is still patrolling waters near the islands.

The CMS began conducting regular patrols near oil and gas projects in China's waters in 2011. It has also sent vessels to stop human activity that may result in environmental damage, according to Li Xinmin, a CMS official.

The rapid development of islands near China's coastal regions has resulted in illegal land reclamation and theft, according to Xie Hongyang, another CMS official.

The CMS determined that 35 development projects had broken laws related to island protection in the first three quarters of last year, resulting in the removal of two projects.