China on Monday kept up the pressure on Japan over the disputed Senkaku islands, sending its coastguard to the area following Beijing's weekend mention of "war" after Tokyo reportedly readied to down its drones.

Four Chinese coastguard vessels sailed into the territorial waters of the islands -- which Beijing calls the Diaoyus -- on Monday morning, the Japanese coastguard said, where they remained for about two hours.

The manoeuvre came days after China, in one of its strongest statements so far in an increasingly acrimonious spat over the islands, 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.

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 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.

Tokyo's top government spokesman, Chief Cabinet Secretary Yoshihide Suga, said Monday: "it is extremely regrettable that intrusions into our territorial waters occur frequently."

Japan annexed what it says were the unclaimed islands in 1895. It says China's assertion of ownership only came after the discovery of resources in the seabed at the close of the 1960s.

Beijing maintains that the islands have been its territory for hundreds of years and were illegally snatched by Tokyo at the start of an acquisitive drive across Asia that culminated in World War II.

The weekend's friction, which came as Japan readied to hold a huge drill intended to sharpen the skills of 34,000 troops in protecting remote islands, further fuelled fears that Asia's two largest economies are skirting dangerously close to conflict.

Observers say the presence of so much hardware in the area increases the risk of an armed confrontation, warning that a slip by a crew member on either side could quickly escalate.