China sent two maritime surveillance vessels to waters close to the Japanese-controlled disputed Senkaku Islands on Tuesday. Xinhua, the country’s official news agency, said the government had devised an action plan and would “conduct missions to safeguard China’s sovereignty depending on the situation”.

The move came in response to an announcement by the Japanese government on Monday that it had reached a deal with the private owner of some of the islands to buy them. While the plan is seen in Japan as a way of cooling tension by preventing the nationalist governor of Tokyo municipality from acquiring the islands, the move has infuriated China, which also claims them.

Before deploying the ships, Beijing had announced territorial baselines for the islands. “This kind of administrative act signals an effort to manage the potential for escalation,” said Taylor Fravel, an expert on Chinese security policy at Massachusetts Institute of Technology. “On the other hand, it could result in more incidents between Chinese and Japanese vessels in the future, as now China’s maritime boundaries in the area are more clearly defined.”

The People’s Liberation Army Daily, the main military newspaper, on Tuesday called Tokyo’s move the most serious provocation against China’s territorial sovereignty since the end of the second world war and warned that while China was committed to peace, it would not pay the price of sacrificing sovereignty and territorial integrity. A spokesman of the ministry of defence later issued a milder statement echoing the foreign ministry’s line that Japan’s move was “illegal” and “invalid”.

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