China has begun work on its first aircraft carrier and probably will develop two or more, along with outfitting a former Russian carrier that is set to begin sea trials soon, Pentagon officials said.

“We expect China to build at least one indigenous carrier, probably two or more, but they have not revealed how many they intend to build, what the construction schedule will [be] or what their missions will be,” said a defense official familiar with intelligence assessments.

A second defense official said China regards aircraft carriers as key symbols of global power projection and is unlikely to build just two.

Other defense officials said assessments about the indigenous carriers are based on intelligence showing construction of the first indigenous carrier at the Changxing Island Shipyard in Shanghai.

The carrier appears in satellite photos to be similar in design to the Varyag, a Soviet-era carrier purchased by China that uses a sky-jump style takeoff ramp at the front of the ship.

The carrier program is one aspect of China’s military buildup that is being closely monitored by U.S. military intelligence.

“We are mindful of China’s military modernization programs and are monitoring carefully this and other military developments in China,” an official said.

The officials commented in response to news last month that China plans to build two indigenous aircraft carriers. Japan’s Yomiuri Shimbun first reported the carrier development on July 10.

The warships are adding to concerns among other Asian nations that fear Beijing will use the power projection platforms to take control of large areas of international waters, like those in the South China Sea, East China Sea and Yellow Sea — where China has claimed maritime sovereignty.

The Chinese military is engaged in a large-scale buildup that includes new strategic and conventional missiles, aircraft, anti-satellite weaponry and a new ballistic missile for targeting ships at sea.

Meanwhile, the U.S. military is facing an Obama administration plan to trim between $400 billion and $1 trillion in defense spending over the next decade.

“Two aircraft carriers are being built at the Jiangnan Shipyard in Shanghai,” a Chinese official with ties to China’s Communist Party leadership told Reuters last week.

China’s tightly controlled state media previously announced no plans for an aircraft carrier force. It had been anticipated since the 1990s when U.S. intelligence agencies detected Chinese jets practicing carrier deck-style takeoffs and landings at inland airfield.

China’s Defense Ministry has confirmed that it will soon begin conducting sea trials on the former Kuznetsov-class carrier called the Varyag, which was purchased several years ago under the guise of converting it into a floating casino.

Defense Ministry spokesman Geng Yansheng told reporters in Beijing on Wednesday that the carrier is old and will be used for “scientific research, experiment and training.”

“The warship is still seaworthy, as it has been docked at sea for some time,” he said, noting that “training for aircraft pilots also in progress.”

Mr. Geng said aircraft carriers are being developed because of China’s long coastlines and large territorial waters.

Regarding future indigenous carriers, Mr. Geng said: “Both overestimation and underestimation of China’s future aircraft carrier have been wrong.”

Building and operating aircraft carriers is a “sacred responsibility of China’s armed forces,” Mr. Geng said, noting the complexity of operating large ships that launch and recover aircraft.

“An aircraft carrier is a weapons platform; it can be used for offensive or defensive purposes. It can also be used to maintain global peace and for rescue and relief work,” he said.

Reports from China said the carrier is expected to begin conducting sea trials this month.

The ship likely will be deployed to the South China Sea island of Hainan, where China also plans to deploy its new nuclear-powered submarines.

China is building attack and ballistic missile submarines, and satellite photos have shown an opening to an underwater submarine base on Hainan.

Tensions remain high in the South China Sea as Chinese ships have moved to assert control over large areas of disputed waters in the sea, which is also claimed by Vietnam, the Philippines, Taiwan, Malaysia and Brunei.

Among those in the region concerned about China’s military buildup is Japan. A government defense report published in December said China’s growing missile and naval forces, and the secrecy surrounding the development, “are of concern for the regional and global community.”

“China’s next moves have to be watched carefully, or there eventually could be a negative impact on maritime safety in Asia,” Yoshihiko Yamada, a professor at Japan’s Tokai University, told Reuters.

China’s interest in developing aircraft carriers increased as a result of the 1996 crisis over Taiwan. At that time, China fired short-range missiles at Taiwan, and the Pentagon responded by sending two aircraft carrier strike groups to waters near the island.