MANILA, Philippines (AFP) - The Philippines' defence secretary on Saturday (March 4) visited a US aircraft carrier patrolling the South China Sea, hailing the "strong relationship" with Washington despite President Rodrigo Duterte's pivot away from Manila's traditional ally.

The US Navy said the USS Carl Vinson had been engaging in "routine operations in the South China Sea" since last month to assert freedom of navigation in the strategic waterway claimed by Beijing, Manila and a host of other Southeast Asian nations.

Defence Secretary Delfin Lorenzana said he and other top officials in Duterte's administration had a "very interesting and informative" visit to the aircraft carrier.

"We have a strong relationship with the US on account of our mutual defence treaty," he added, referring to the cornerstone 1951 agreement.

Duterte has thrown the Philippines' 70-year-old alliance with the US into question, threatening a "separation" from Washington last year and calling for the withdrawal of American troops from his country.

After angrily rejecting US criticism of his deadly war on drugs, the firebrand president has shifted his nation's foreign policy more towards China and Russia.

Duterte has made overtures to Beijing despite a long-running territorial dispute over the South China Sea.

Lorenzana said the Philippines had "nothing to do" with America's naval patrols through Chinese-claimed waters but said he was "impressed" by the aircraft carrier and its some 70 aircraft.

"They have more combat planes than the (Philippine Air Force)," he told AFP.

Under previous governments, Manila tried to improve defence ties with its former colonial ruler Washington to boost one of Asia's weakest armed forces.

Duterte has said he plans to source weapons from Beijing and Moscow instead.