Japan has unveiled its biggest warship since the Second World War, leading to fears that the conflict over disputed islands between it and China could escalate.

The 19,500-tonne behemoth, described as a destroyer but with the capacity to carry up to 14 helicopters on its 250m (820ft) long flight deck, has also been criticised as a disguised attempt to boost the country’s offensive military capabilities – strictly limited by its constitution.

Japanese officials said the primary function of the ship would be in national defence. It is specially designed for anti-submarine warfare, and is expected to play an integral role in border surveillance.

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The ship, which cost $1.2 billion, is also intended to be capable of assisting in humanitarian response missions. It will be used to transport evacuees, emergency services personnel and supplies in the event of large-scale natural disasters like the earthquake and tsunami of 2011.

But many see the unveiling of the destroyer now – dubbed “Izumo” – as a direct response to the disputed land between Japan and Taiwan. The islands, called the Senkaku in Japan and the Diaoyutai in China, have for months seen almost constant patrols by ships from both countries.

China recently began operating its own aircraft carrier in the area, bought from Russia and re-fitted for the purpose. And seemingly in response to the construction of Izumo, China says it is moving forward with plans to build a brand new massive warship for its own fleet.

Great efforts were made at the unveiling of the new ship in Japan to emphasise its defensive and humanitarian purposes, partly because the country’s constitution prohibits it from establishing any offensive military capabilities.

Some experts have said that Izumo’s close resemblance to an aircraft carrier could see it easily adjusted in the future to accommodate fighter jets – deemed offensive weaponry. While it has one of the best equipped and best trained naval forces in the Pacific, Japan said it would be making no such ventures into active or aggressive warfare.

The unveiling of Izumo comes after controversial comments made by the country’s finance minister, Taro Aso, in which he was reported as saying Japan should emulate the Nazis by subtly revising the constitution, in part to allow for a higher-profile military, without anyone noticing.