Japanese Vice Foreign Minister Chikao Kawai, the highest-ranking foreign ministry bureaucrat, summoned Chinese ambassador Cheng Yunghua to complain about the intrusion of patrol ships.

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Japan also appears to be looking to reinforce its capabilities in the area - public broadcaster NHK reported on Monday that two more patrol ships will be deployed to the area later in the year.

BBC: China 'to survey disputed East China Sea islands'

The Americans have their own ships on patrol, as do the Chinese, the Indians, Russians and several other countries.

ECONOMIST: Pirates kidnap 20 American sailors off the coast of Somalia

Up to 10 EU naval ships are currently on patrol off the Horn of Africa.

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"There aren't enough ships on the planet to patrol the entire Indian Ocean, " she said, adding that she thought it would be more difficult to eradicate Somali piracy than to bring peace to Afghanistan.

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On Thursday China said that it granted its border patrol police the power to board and search ships in the area.

BBC: File photo: Surin Pitsuwan

At present, warships from a number of countries patrol an international maritime corridor, escorting some merchant ships and responding to distress calls in the area.

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In addition, seven Royal Fleet Auxiliary ships and four support aircraft, consisting of Nimrod maritime patrol and Hercules transport planes will be part of the force.

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The ships, which left Hainan province on 19 March, "conducted patrol and training missions" over eight days, it added.

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