Bullit Marquez, Associated Press
In this April 20, 2015 file photo, Armed Forces of the Philippines Chief of Staff Gen. Gregorio Pio Catapang, left, points to reveal recent images of China's reclamation activities being done at the disputed islands in the South China Sea during a news conference at Camp Aguinaldo at suburban Quezon city, northeast of Manila, Philippines.

WASHINGTON — China's rapidly expanding campaign to construct artificial islands, potentially for military use or airstrips, now totals about 2,000 acres and could far outstrip that amount as the year goes on, a senior defense official said Friday.

The new estimate, disclosed for the first time, comes as the Pentagon released its annual report on China's military power. The report warns that while Beijing's island construction intent is unclear, five emerging outposts could be used for surveillance systems, harbors, an airfield and logistical support.

The wide-ranging report also details Chinese government-backed cyberattacks against the U.S. government to collect intelligence and steal high-tech data from defense programs to support China's industry. And it warns that China now "boasts the most dynamic space program in the world today."

The document also repeats persistent U.S. concerns about China's growing capacity to project military power beyond its borders, with continued investments in new missiles, ships, and aircraft as well as cyber, space and electronic warfare.

For the first time, however, the report describes the "extensive" land reclamation at five outposts in the Spratly Islands, adding that at four of the sites the work has shifted to mainly development and construction.

The report says China had reclaimed about 500 acres as of late last December, underscoring the dramatic increase in the past four months. The defense official said the construction has been methodical and continuous since December. The official was not authorized to discuss the issue publicly and spoke on condition of anonymity.

According to the report, China's government says the projects "are mainly for improving the living and working conditions of those stationed on the islands. However, most analysts outside China believe that China is attempting to change facts on the ground by improving its defense infrastructure in the South China Sea."

Although it is unclear what will ultimately be built on these expanded outposts, the report says they could include harbors, communications and surveillance systems, logistics support, and at least one airfield.

China's reclamation program in disputed waters of the South China Sea dwarfs that of any other nation and has inflamed tensions in the region. Last month, the Philippines urged its fellow Southeast Asian countries to take immediate steps to halt the building, warning that failure to do so will allow Beijing take "de facto control" of the area.

According to the defense official, Vietnam has reclaimed about 60 acres of land since 2009. And the report said that Taiwan began a "modest land reclamation effort" on Itu Aba Island last year, and has reclaimed about five acres near the island's airstrip. There are reports that Taiwan is building a $100 million port near the airstrip that could accommodate naval frigates and coast guard cutters, the Pentagon said.