Japan warned China on Wednesday that its extensive land reclamation in the disputed South China Sea does not make ownership "a done deal", after Beijing announced it had almost finished its controversial island-building.

The rebuke came after Washington urged China against militarisation of the area, saying it risked escalating tensions, even as satellite pictures have shown a runway long enough to let even the biggest aircraft land.

It also came as details emerged of a joint exercise between Japan and the Philippines, as the relationship blossoms between the two regional powers most prepared to push back against Beijing's perceived rising aggression.

"We hold serious and significant concerns about the unilateral actions aimed at changing the status quo, which are bound to increase tension," the Japanese government's top spokesman Yoshihide Suga told reporters.

"With the completion of the reclamation, we must not accept the land reclamation as a done deal. We demand (China) not take unilateral actions that bring irreversible and physical changes," he added.

Responding to the comments, Beijing countered that "it makes no sense" for Japan to press China on the issue.

"China has indisputable sovereignty over the Nansha Islands and we do not need to prove this by building facilities on the islands and reefs," foreign ministry spokesman Lu Kang told a regular press briefing, using the Chinese name for the Spratly Islands in the South China Sea.

Japan has long criticised China's attempts to change the status quo unilaterally and by force, mindful of its own dispute with Beijing over islands in the East China Sea.

The US says China has created 2,000 acres (800 hectares) of new land in the South China Sea in the last 18 months.

Huge dredgers have been spotted dumping sand on previously submerged reefs, many of which now house buildings and ports.

Manila said Wednesday that a three-kilometre (1.9-mile) runway on Fiery Cross Reef -- big enough to handle a Boeing 747 -- was "75 percent complete".

"This can serve as China's forward operating base, a refuelling stop for ships and aircraft," Philippine defence department spokesman Peter Galvez told AFP.

"This will allow China easy reach in the West Philippine Sea (the Filipino name for a section of the South China Sea claimed by Manila) and extend their reach up to Australia and other parts of the South Pacific."

"They can do anything they want there. It could be their command and control centre," he added.

- 'Militarisation' -

Beijing claims almost all the South China Sea, despite a number of overlapping territorial claims by the Philippines, Vietnam, Malaysia, Brunei and Taiwan.

Some commentators suspect China is preparing to declare an Air Defence Identification Zone over the sea, forcing all aircraft to declare themselves to Chinese authorities -- after a similar move in the East China Sea in 2013.

The US is particularly wary of Beijing's growing ambitions in the area, and last month invited a television crew aboard a surveillance plane as it flew near the island reclamation work.

Chinese military transmissions could be heard telling the American plane to stay away.

Washington on Tuesday welcomed Beijing's announcement of an end to some reclamation work, but warned against weaponising the new islands.

"We certainly don't want to see the militarisation of these facilities," a senior State Department official told reporters. "They do nothing to decrease tensions in the region, in fact they have the opposite effect."

"China is alone on this. There's nobody else in the region that's supportive of these efforts."

The Philippines has led Southeast Asia's charge against China, with President Benigno Aquino likening its actions to those of Nazi Germany in the run-up to World War II.

Manila has also sought support from the US and from Japan, including in acquiring military and paramilitary hardware to bolster its woefully-equipped coastguard, which is at the frontline of its pushback.

Japan and the Philippines will hold their second joint naval drill next week in waters near the contested Spratly islands, following an exercise in the South China Sea in May.

Japan said the mission, scheduled for June 21 through 27 in waters around and northwest of the island of Palawan, will involve a Japanese P-3C surveillance plane, and a plane and patrol ship from the Philippine navy.

On May 12 two Japanese destroyers and one of the Philippines' newest warships held their first drill less than 300 kilometres from the Philippine-claimed Scarborough Shoal, which is now under Chinese control.