Beijing kept up the pressure on Tokyo over the Diaoyu Islands yesterday, sending its coastguard near the disputed island chain following a weekend of sabre-rattling by both sides.
Four Chinese coastguard vessels sailed into the territorial waters around the Diaoyus, which the Japanese call the Senkakus, the Japanese coastguard said. The ships remained in the area for about two hours.
The patrol came days after China, in one of the strongest statements of the increasingly acrimonious spat over the islands, said that any firing on its unmanned aircraft "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.
Earlier, Japanese Prime Minister Shinzo Abe had invoked support for a robust regional response to what he said was Beijing's attempt to "change the status quo by force".
The patrols 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.
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.
Chinese ships have sailed into the waters around the Senkakus dozens of times since Japan purchased three of the islands last September.
Usually, the ships stay for a few hours before returning to the contiguous zone outside territorial waters.
Japan's top government spokesman, Chief Cabinet Secretary Yoshihide Suga, said on Monday: "It is extremely regrettable that intrusions into our territorial waters occur frequently."