China on Tuesday returned a US underwater probe it seized in the South China Sea, the Pentagon confirmed after Beijing's capture of the craft sparked a dispute between the two powers.

The Chinese navy handed over the drone near where it was seized, the Pentagon said, repeating US condemnation of Beijing's actions in what it says are international waters.

"This incident was inconsistent with both international law and standards of professionalism for conduct between navies at sea," Pentagon press secretary Peter Cook said in a statement.

We have "called on Chinese authorities to comply with their obligations under international law and to refrain from further efforts to impede lawful US activities."

A US defense official, speaking on condition of anonymity, told AFP the probe was handed over to the guided-missile destroyer USS Mustin and will be taken to a naval base for inspection.

A Chinese naval vessel seized the probe last week around 50 nautical miles northwest of Subic Bay in the Philippines, a move that heightened existing tensions between the world's two largest economies.

The Pentagon statement said the US Navy drone was "conducting routine operations in the international waters of the South China Sea in full compliance with international law."

- Rise in 'interactions' -

For its part, China said the handover of the drone was "completed smoothly" after "friendly consultations" between both sides, according to a short defense ministry statement on its website.

Foreign ministry spokeswoman Hua Chunying said the handling of the incident showed the two countries have a "smooth channel of communication."

But she also warned the US against "conducting close reconnaissance in China's coastal waters."

"China is strongly opposed to this and has been asking the US to stop these kinds of activities," she said, adding: "I believe this was the root cause for this incident happening."

A second US defense official told AFP there had been an increase in "interactions" between US and Chinese vessels over the past year in the South China Sea and western Pacific.

Pentagon officials said last week the Chinese had "unlawfully" grabbed the marine probe, which they described as a craft that gathers unclassified data -- including water temperatures, salinity and sea clarity.

Such data can be used to help submarines navigate and determine sonar ranges in murky waters.

China said it snatched the craft because it might pose a safety hazard to other vessels. It also said it "strongly opposed" US reconnaissance activities and had asked Washington to stop.

Washington insists the small, slow-moving craft cannot be used for surveillance.

- Continuing tensions -

The incident has heightened continuing tensions in the South China Sea. Beijing has fortified its claims to almost all the waterway by expanding tiny reefs and islets into artificial islands hosting military facilities.

Brunei, Malaysia, the Philippines, Vietnam and Taiwan have competing claims in the waterway.

While Washington takes no position on the sovereignty disputes, it has repeatedly called on China to uphold freedom of navigation.

Its military has conducted several operations in which ships and planes have passed near the sites Beijing claims.

US President-elect Donald Trump raised the rhetorical heat further after the probe was seized, by accusing Beijing of theft.

After Beijing and Washington announced on Sunday the drone would be returned, he tweeted: "We should tell China that we don't want the drone they stole back. - let them keep it!"

China's foreign ministry on Monday rejected Trump's accusations Beijing had stolen the craft as "not accurate".

The state-owned China Daily said in an editorial earlier that Trump's behavior "could easily drive China-US relations into what (US President Barack) Obama portrays as 'full-conflict mode.'"

Trump had already angered China by questioning longstanding US policy on Taiwan, calling Beijing a currency manipulator and threatening punitive tariffs on Chinese imports.

Though this is the first time the Chinese navy has seized a probe, it is not unusual for the underwater craft to go missing.

The Navy deploys about 10 ocean gliders in the western Pacific each month, and a small percentage of these are lost in fishing nets, storm damage or through equipment failures.

"In October, one of our ocean gliders was lost in the vicinity of Vietnam, but we are not sure of the final disposition of the glider," said Lieutenant Commander Matt Knight, a US Pacific Fleet spokesman.