China has kept up the pressure on Japan over disputed islands, sending its coastguard to the area following Beijing's weekend mention of "war" after Tokyo reportedly readied to down its drones.
In one of its strongest statements so far in an increasingly acrimonious spat over the islands, Beijing said if Japan fired on its unmanned aircraft it "would constitute a serious provocation, an act of war of sorts".
"We would have to take firm countermeasures, and all consequences would be the responsibility of the side that caused the provocation," China's defence ministry said.
Those comments, published Saturday, came after Japanese Prime Minister Shinzo Abe invoked support in Asia for a robust response to what he said was Beijing's attempt to "change the status quo by force".
They also followed reports that Abe had given the okay to a plan to shoot down drones entering Japanese airspace if they did not heed warnings to leave.
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On Sunday he told troops the "security environment surrounding Japan is becoming increasingly severe".
"You will have to completely rid yourselves of the conventional notion that just the existence of a defence force could act as a deterrent."
Reports said that Japan scrambled fighter jets on Sunday for the third consecutive day, as Chinese military aircraft overflew a strait between two Okinawan islands. They did not enter Japanese airspace.
On Monday, four Chinese coastguard vessels sailed into the territorial waters of the Senkaku islands - which Beijing calls the Diaoyus - shortly before 10am (1200 AEDT), the Japanese coastguard said.
Chinese ships have done this dozens of times since Japan nationalised three of the islands in September 2012. Each time they trade warnings and claims of sovereignty with their Japanese opposite numbers.
Customarily, they stay for a few hours and then move out into the contiguous zone, a band that, under international definitions, lies outside the 12-nautical-mile territorial waters.
