China is expected to add substantial military infrastructure, including surveillance systems, to artificial islands in the South China Sea this year, giving it long-term "civil-military bases" in the contested waters, the Pentagon said on Friday, May 13, according to Reuters.

In its annual report to Congress on China's military activities in 2015, the U.S. Defense Department estimated that China's reclamation work had added more than 3,200 acres (1,300 hectares) of land on seven features it occupied in the Spratly Islands in the space of two years.

It said China had completed its major reclamation efforts in October, switching focus to infrastructure development, including three 3,000 meter-long airstrips that can accommodate advanced fighter jets, Reuters says.

"Additional substantial infrastructure, including communications and surveillance systems, is expected to be built on these features in the coming year," the report said.

"China will be able to use its reclaimed features as persistent civil-military bases to enhance its presence in the South China Sea significantly."

The report comes at a time of heightened tension over maritime territories claimed by China and disputed by several Asian nations. Washington has accused Beijing of militarizing the South China Sea while Beijing, in turn, has criticized increased U.S. naval patrols and exercises in Asia.

The Pentagon report said China was focusing on developing capabilities to counter outside intervention in any conflict, but appeared to want to avoid direct confrontation with the United States in Asia, given the potential economic damage, Reuters says.

At the same time, "China demonstrated a willingness to tolerate higher levels of tension in the pursuit of its interests, especially in pursuit of its territorial claims," the report said.