China said Tuesday it would soon complete some of its land reclamation projects on disputed islands in the South China Sea, but that infrastructure work there would continue.

Chinese island-building in the strategic waters has alarmed its neighbours and fuelled tension between the Chinese and the US military around the Spratly Islands.

"As scheduled, China will complete its reclamation projects on some stationed islands and reefs of the Nansha Islands in a short time," a foreign ministry statement said, using the Chinese name for the Spratlys.

It did not say when precisely the work would end but described the work as "lawful, reasonable, sensible".

China, which regards almost all the South China Sea as its own, has intensified its building of artificial islands and facilities including airstrips in the contested waters in recent months.

After the completion of the reclamation projects, China would undertake "facility construction" to meet "relevant functions", said the statement, which did not specify which projects are nearing completion.

The South China Sea -- which is believed to be home to important oil and gas reserves -- is a major shipping route for international trade.

The Chinese military last month ordered a US Navy P-8 Poseidon surveillance plane to leave an area above the Spratlys.

But the American aircraft ignored the demand and said it was flying in international airspace.

US Defense Secretary Ashton Carter said last month that while other countries had also established outposts, China had reclaimed over 2,000 acres (809 hectares) in the past 18 months -- more than all other claimants combined.