China’s Defense Ministry announced on Saturday that it had been deferring with the United States about returning an underwater drone previously reported to have been taken by a Chinese vessel in the South China Sea, but the U.S. was not helping by “hyping up” the issue.

U.S. President-elect Donald Trump weighed in to the row on Saturday, tweeting: “China steals United States Navy research drone in international waters – rips it out of water and takes it to China in unprecedented act.”

The drone was taken on Thursday, the first seizure of its kind in recent memory. The Defense Ministry said a Chinese naval vessel discovered a piece of “unidentified equipment” and checked it to prevent any navigational safety issues, before discovering it was a U.S. drone.

“China decided to return it to the U.S. side in an appropriate manner, and China and the U.S. have all along been in communication about it,” the ministry said on its website.

“During this process, the U.S. side’s unilateral and open hyping up is inappropriate, and is not beneficial to the smooth resolution of this issue. We express regret at this,” it added.

Without directly saying whether the drone was operating in waters China considers its own, the ministry said U.S. ships and aircraft have for a long period been carrying out surveillance and surveys in “the presence” of Chinese waters.

China will remain on alert for these sorts of activities and take necessary steps to deal with them, the ministry said without elaborating.