The presence of the Nimitz in the South China Sea comes amid tensions between China and the Philippines, an American ally, over Scarborough shoal. There coral reefs enclose a large lagoon about 135 nautical miles (250km) west of Manila Bay,

The Chinese military said it deployed naval vessels to protect international sea lanes and maintain regional peace and stability.

Navy helicopter and fighter jet from USS Nimitz crash in South China Sea during routine operations. All five crew members successfully rescued and stable.

All personnel involved were safe, and the causes of the two crashes were under investigation, the U.S. Navy's Pacific Fleet said.

The chief of the Philippine military said the goal was to hold the line for "at least 20 days" in a wartime scenario.

China's ambitious five-year strategic blueprint has revealed the country's plan to become a global sea power, with pledges to secure its maritime periphery while countering American sanctions and other perceived external security threats.

The contested South China Sea boasts rich oil and gas reserves and could play a key role in a conflict over Taiwan.

The move, if it occurs, would be in response to China's escalating hostilities near disputed waters as Trump is poised to meet with Xi, sources say.

A helicopter and a fighter jet went down in separate incidents while operating from a U.S. aircraft carrier on Sunday, the Navy said. The crews were rescued.