Washington, Dec 18 (IANS): China has agreed to return the US underwater drone that it seized in the international waters last week, the Pentagon said, reports said on Sunday.

"We have registered our objection to China's unlawful seizure of a US unmanned underwater vehicle operating... in the South China Sea. Through direct engagement with Chinese authorities," Pentagon spokesman Peter Cook said in a statement.

"We have secured an understanding that the Chinese will return the UUV to the US," CNN quoted Cook's Saturday statement.

Details were not immediately available on when or how the drone might be returned.

Earlier, China's defence ministry said authorities decided on the return, but criticised the US for having "hyped up" the issue.

President-elect Donald Trump had weighed in on the incident, calling China's action "unprecedented".

Initially Trump tweeted, "China steals United States Navy research drone in international waters - rips it out of water and takes it to China in unprecedented act."

Later he said, "We should tell China that we don't want the drone they stole back.- let them keep it!"

"Upon confirming that the device was a US underwater drone, the Chinese side decided to transfer it to the US side in an appropriate manner," Chinese Defence Ministry spokesman Sr Col Yang Yujun said.

"China and the US have been communicating about this process. It is inappropriate -- and unhelpful for a resolution -- that the US has unilaterally hyped up the issue. We express our regret over that."