China's mysterious campaign to build artificial islands in disputed waters of the South China Sea has grown to cover roughly 2,000 acres, a senior defense official said Friday, as the Pentagon issued a report warning the sites could be used to bolster the country's military infrastructure. 

The assessments provide an unprecedented level of detail on what China is suspected of pursuing with its so-called "land reclamation" project, which the Defense Department now describes as "extensive." 

The U.S. military initially called out China for the project in March, explaining that the country was "pumping sand on to live coral reefs" and then paving over them with concrete. 

The construction, though, has apparently proceeded at a rapid and alarming pace. The new Defense Department report on China's military activities said that as of late December, China had reclaimed about 500 acres of land in this way. 

A senior defense official told the Associated Press on Friday that the land mass has since grown to about 2,000 acres and could keep growing as the year goes on. 

The DOD report warned that while Beijing's island construction intent is unclear, five emerging outposts in the Spratly Islands could be used for surveillance systems, harbors and logistical support. The report said they could also hold "at least one airfield." According to the Defense Department, work at four of the five sites has shifted to mainly development and construction. 

The report described how China has "excavated deep channels and built new berthing areas to allow access for larger ships to the outposts." 

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." 

The report stressed that the artificial islands do not provide China "with any additional right to claim maritime geography within the South China Sea," while acknowledging China "will be able to use them as persistent civil-military bases of operation to significantly enhance its presence in the disputed area." 

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. 

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. 

The Associated Press contributed to this report.