As the commanders of the largest U.S. warship in Asia seek to maintain operational readiness amid protracted tensions over North Korea, they find themselves keeping one eye on China, too.

On Saturday, as F-18 Super Hornet jet fighters roared from the decks of the USS Ronald Reagan aircraft carrier during routine drills deep in the South China Sea, two Chinese frigates maintained a constant line-of-sight vigil.

Officers on the Japanese-based Reagan described frequent close quarter surveillance from the ships of the People's Liberation Army Navy in international waters.

Sometimes, they said, Chinese vessels steam in to check out the carrier en route to other destinations. Other times, Chinese frigates linger for days within the screen of U.S. ships and planes that protect the Reagan - Washington's only carrier based outside America.

At times, the carrier crew, to ensure safe passage, will alert their uninvited Chinese escorts, should the Reagan sharply alter course, officers said.

"We’ve had no issues. They've been very professional," said Rear Admiral Marc Dalton, commander of the Reagan's strike group, as well as the larger battle forces of the U.S. Seventh Fleet. "We see them on a regular basis," he said.

DEPLOYMENT STRAINS

As Dalton spoke, the midnight blue waters beyond the flight decks made for a crowded scene, with a U.S. and an allied Japanese destroyer also visible as the Reagan manouvered some 400 nautical miles (748 kilometres) from the Chinese coast.

It provided a window into the strains of increased deployments and exercises by regional militaries, in part as they respond to the threat posed by Pyongyang's pursuit of nuclear and missile programmes.

North Korea has staged an apparent hydrogen bomb test and fired two ballistic missiles over Japan in recent weeks.

The situation has unfolded as U.S. naval chiefs review operations to draw lessons from four significant accidents within the U.S. Pacific Fleet this year.

A report this month by the U.S. Government Accountability Office highlighted a host of training and maintenance problems as the navy strove to expand overseas deployments and improve operational readiness.

Dalton will soon lead the carrier to a port call in Chinese-ruled Hong Kong - the first such visit in three years after a stop by another carrier was denied in 2016.

China, which claims much of the disputed South China Sea, has long objected to U.S. military operations off its coasts, even in areas Washington insists are free to international passage.

DOMINANT PRESENCE

Routinely carrying between 60-70 aircraft on board, the carrier sends between 80-100 sorties daily - the core of a dominant U.S. military presence in Asia that analysts believe China could still take years to supplant.

In recent months, the 100,000-ton Reagan has exercised with allied Australian ships as well as Japanese forces.

The South Korean defence ministry has announced it will exercise with the Reagan strike group in October.

Rear Admiral Dalton acknowledged the challenges and top-level reviews but insisted the task force was long used to keeping itself in full readiness.

"As a forward deployed force. . . we are already where we need to be to execute our missions all the time," he said.

He did not detail any specific North Korean contingencies but described Pyongyang's missile tests and nuclear programme as a "growing and concerning danger".

"The U.S. has been very clear about leveraging all options in order to get North Korea to change its path," he said.