The United States and the Philippines held their third joint sea and air patrol in three months off the Southeast Asian country's coast on Friday, the allies said.

The latest sea and air patrols in the South China Sea come as Washington seeks to strengthen its defence ties in the region in the face of an increasingly assertive China.

In separate exercises, the US and Japanese navies also held joint drills in the Philippine Sea this month in a show of force as tensions with China and North Korea rise.

Washington, which has a mutual defence pact with Manila, has denounced aggressive Chinese actions towards its ally and vowed to keep open the South China Sea, through which trillions of dollars worth of trade is shipped every year. 

Beijing claims almost the entire waterway and has ignored an international ruling that its claims have no legal basis.

China also claims self-ruled Taiwan as its own and has vowed to seize the island, by force if necessary, ramping up pressure with major military drills in the Taiwan Strait.

Friday's sea and air patrols involved the US combat ship USS Gabrielle Giffords and an MH-60S Sea Hawk helicopter, along with the Philippine Navy's BRP Gregorio del Pilar warship and an AW109 helicopter.

The Philippine military said the patrol was held in its exclusive economic zone but did not provide an exact location.

"Sailing and operating together demonstrates our commitment to improving our interoperability and continued coordination," Captain Sean Lewis, commodore of US Destroyer Squadron 7, said in a statement.

Philippine military chief General Romeo Brawner said the patrol "fosters closer cooperation towards further enhancing our maritime capabilities".

Their first joint sea and air patrol was in November and was followed by a second round in January, as China held military drills in the same waters.

That came weeks after tense standoffs between China and the Philippines around disputed reefs in the South China Sea that saw a collision between vessels from the two countries and Chinese ships blasting water cannon at Philippine boats.