China's rapidly expanding campaign to construct artificial islands, potentially for military use or airstrips, now totals about 2,000 acres. That's according to a senior defense official.

The reclamation could far outstrip that amount as the year goes on.

It's the first time officials have released the total. The new details come as the Pentagon gives an annual report on China's military power. The report warns that while Beijing's intent is unclear, the five emerging outposts could be used for surveillance systems, harbors, an airfield and logistical support.

The report says China had reclaimed about 500 acres as of late last December. The defense official says building has been methodical and continuous since then. The official was not authorized to discuss the issue publicly and spoke on condition of anonymity.