Beijing on Thursday rejected Pentagon accusations that a Chinese aircraft made an "unsafe" intercept of a spy plane in international air space, as tensions rise in the strategically vital South China Sea.

Rivalry between China and the United States is mounting in the disputed waterway, an important shipping route thought to be home to vast energy deposits, and which Beijing claims almost in its entirety.

The US Department of Defense (DoD) told reporters Wednesday that two Chinese tactical aircraft intercepted an American reconnaissance plane in an "unsafe" manner, without giving details.

China's foreign ministry spokesman Hong Lei said Thursday the description was "untrue".

Two Chinese aircraft tailed a US EP-3 reconnaissance plan as it "flew close" to the island province of Hainan, he said, but kept "a safe distance" and did not make "dangerous moves".

Such US flights were a "severe threat" to Chinese security, he added, calling for Washington to stop them immediately.

The incident comes more than a decade after a collision between a Chinese fighter jet and a US Navy EP-3 which killed the Chinese pilot and forced the US aircraft to make an emergency landing on Hainan.

A 11-day standoff ensued as Beijing interrogated the 24 US crew, seriously straining relations between the countries, and China went on to hold the plane for several months.

The two have traded accusations and warnings over such surveillance flights in subsequent years.

- Rival claims -

Beijing has been building islets in the South China Sea into artificial islands with military facilities including radar systems and airstrips.

Regional neighbours such as Vietnam and the Philippines have rival claims and the United States says China's assertions have no basis in law.

Washington -- which has embarked on a foreign policy "pivot" towards Asia -- fears Beijing is seeking to impose military controls over the entire area.

Much to Beijing's annoyance, the US military has conducted several "freedom of navigation" operations, in which planes or ships pass within a 12-nautical-mile buffer around the Chinese installations.

The latest intercept came after the Pentagon and China had worked to reduce the risk of mishaps "by improved dialogue at multiple levels".

"Over the past year, DoD has seen improvements in PRC actions, flying in a safe and professional manner," the Pentagon spokesman said Wednesday, adding that the Department of Defense was "addressing the (intercept) through the appropriate diplomatic and military channels".

The encounter also came days after China accused the US of distorting facts in a report on the Asian giant's defence policy and warned Washington it had "severely damaged" trust between the superpowers.

In the annual report to Congress the Pentagon said Beijing had been using "coercive tactics" to assert its claims in the South China Sea.

The Pentagon report estimated China has reclaimed 3,200 acres (1,300 hectares) of land in the Spratly Islands, also claimed by the Philippines, over the past two years.

Beijing has been angered by the growing US attention on Asia and American forays into the Sea, including sailing warships close to reclaimed islands.

"It is the United States that has been flexing military muscles by frequently sending military aircraft and warships to the region," a Chinese defence ministry spokesman told state media following the report.