The rebuke came after Washington urged China against militarisation of the area, saying that 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.
"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 said.
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.
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The US says China has created 2,000 acres (800 hectares) of new land in the South China Sea in the last 18 months.
Manila said today that a three-kilometer (1.9-mile) runway on Fiery Cross Reef - big enough to handle a Boeing 747 - was "75 per cent 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 term 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."
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.
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.