Image copyright People's Daily Image caption Official media said the US was a "repeat offender" in the South China Sea

China's media are expressing heightened concerns at relations with the US, despite the Pentagon securing "an understanding" that China would return an underwater drone seized in the South China Sea last Thursday.

Official Chinese media say the US marine presence there is the behaviour of a "repeat offender", and criticise President-elect Donald Trump for his outspoken social media comments on the incident.

They hint that China will take a much heavier-handed approach once Mr Trump becomes president in January.

Chinese state media have long accused the US of "interfering" in the South China Sea, over which China claims full sovereignty.

'Repeat offender'

China's leading paper for foreign affairs, Global Times, praises the Chinese navy for "identifying and inspecting" the drone, and dubs US surveillance in the region "typically hegemonic behaviour" and "bullying".

Image copyright US Navy Image caption The USNS Bowditch was in the region to conduct research

The overseas edition of People's Daily describes the USNS Bowditch, which released the unmanned drone, as a "repeat offender... that has conducted many surveillance operations on China".

It says this latest incident "shows the US remains questioning or even hostile towards China" and risks "barring mutual trust".

'May treat relations as a game'

The media in particular highlight Mr Trump's social media comments.

Although Twitter is blocked in China, media, including the English-language paper China Daily, shared screenshots of Mr Trump's 17 December post, including his grammatical error that the drone was stolen from the US in an "unpresidented act".

Image copyright Global Times / Sina Weibo Image caption Global Times said that Donald Trump doesn't "look like a president-elect"

Global Times says it fails to "see a sense of responsibility" in his comments, adding "Trump doesn't look like a president-elect".

"If these are remarks from media commentators or simply social media users trading abuse, they would be understandable," it says.

"But even the US military has not used the word 'steal' to describe China's move. Trump's tone... shows he isn't afraid to hype-up the issue - this raises concerns that he may treat relations as a game between the two major countries."

'China is being restrained'

The press suggests that China will go harder on Mr Trump once he takes over in January.

Image caption In July, China's media opposed a Hague ruling that the South China Sea was not sovereign Chinese territory

China Daily says that, although an understanding "through direct engagement, as the Pentagon put it", has been reached, "Trump may not care for such an ending."

"It is good for both countries that Trump is still commander-in-waiting, or we might have a situation where cooler heads are pushed aside by surging emotion," it says.

Global Times also warns that China may take a firmer approach come January.

"China is being restrained in its response towards Trump's words, because he hasn't officially taken office yet," it says.

"But such an attitude will certainly change after Trump is in the White House."

'Picking up the rubbish'

On social media, a strongly worded post via the military page on the popular Sina web portal prompted thousands of angry comments.

Image copyright Gudabaihua / Sina Weibo Image caption User Guda Baihua's share of Mr Trump's tweet ignited heated online discussion

It questioned US "moral integrity", saying the presence of the vessel left them "stunned".

Users voiced their approval of the Chinese navy for having removed the drone. "Onlyzyz" says: "The US supports environmental protection, so we are just picking up the rubbish!"

Thousands also commented on Mr Trump's remarks, which popular user "Guda Baihua" shared.

"Vee1119" says Mr Trump should "open an account on Weibo", China's popular micro-blogging site, to hear the Chinese public's views.

BBC Monitoring reports and analyses news from TV, radio, web and print media around the world. You can follow BBC Monitoring on Twitter and Facebook.