Two Chinese fighters have conducted an "unsafe" intercept of a US spy plane in international air space over the South China Sea, the Pentagon said, as tensions mount in the strategically vital waters.

Tensions between China and the United States are high 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 Department of Defense (DoD) is reviewing a May 17 intercept of a US maritime patrol reconnaissance aircraft by two tactical aircraft from the People's Republic of China (PRC)," Pentagon spokesman Major Jamie Davis said in a statement Wednesday.

"Initial reports characterized the incident as unsafe," he added, without giving additional details.

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

The crash, which occurred in 2001, unleashed an 11-day standoff as Beijing interrogated the 24 US crew, and held the plane for several months, seriously straining relations between the countries.

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

The Chinese defense ministry said in a statement faxed to AFP Thursday that they "noted" reports of the latest incident and said it "is very likely linked to the extremely close surveillance of China by US military aircraft".

- Rival claims -

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

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

Washington -- which is 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," Davis said.

"DoD is 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 defense 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 defense ministry spokesman told state media following the report.