The European Union's ambassador to China said Wednesday it was "deeply concerned" about Beijing's detention of EU citizens, hours after state media broadcast the purported confession of a Swedish activist held by authorities.

Peter Dahlin, who worked for the Chinese Urgent Action Working Group, disappeared earlier this month as he prepared to board a flight to Thailand, and appears to have been caught up in a crackdown on human rights lawyers.

His detention comes as China considers a new law to control the activities of foreign non-governmental organisations (NGOs), which has raised widespread concern among overseas groups.

Dahlin's group has said it offered training to lawyers who have tried to use China's tightly-controlled judiciary to redress apparent government abuses.

Late Tuesday state broadcaster China Central Television showed a video of the Swede confessing that he had "violated Chinese law through my activities here".

"I have hurt the feelings of the Chinese people," he declared in stilted tones. "I apologise sincerely for this."

"I have no complaints to make," he added. "I think my treatment has been fair."

Two purported Chinese colleagues of Dahlin were also shown declaring their guilt.

The official Xinhua news agency said Dahlin's group was "encouraging the masses to oppose the government". Foreign ministry spokesman Hong Lei said Wednesday the Swede "took part in activities in China which threatened state security".

It is rare for China to accuse foreigners of national security offences, which can carry heavy penalties, although some have been accused of spying.

The EU was "deeply concerned" about cases such as Dahlin's, the grouping's ambassador to China Hans-Dietmar Schweisgut told reporters.

"We do hope it's not representing the new normal yet, but we do see an extremely worrying trend," he said.

- Scaring donors -

Overseas organisations ranging from human rights groups to business councils have strongly criticised China's proposed legislation on foreign NGOs, fearing it will effectively make it impossible for them to work in the country.

"We are concerned that this could have an impact on overall economic, business relations or academic exchanges", Schweisgut said, adding that it "goes much further than human rights".

"This is a serious and big issue", he said.

As China tightened social controls in recent years, it has sought to blame "foreign forces" for domestic woes, ranging from undermining Chinese values to a recent explosion in labour protests.

As the NGO law moves towards being passed, "foreign funders will be increasingly portrayed as the 'black hands' behind China's rights activists", Maya Wang, a campaigner at Human Rights Watch, told AFP.

Airing Dahlin's confession -- in which he admitted funding Chinese lawyers -- probably had two "benefits" for the government, she said: "Smearing and building a fake criminal case" against detained rights lawyers and "scaring donors away from China's human rights activists".

Authorities launched a sweeping crackdown on human rights attorneys in July, detaining more than 130 legal staff across the country. This month at least 10 were formally arrested on charges related to "state subversion" after being held in secret for six months.

"Whatever Peter Dahlin did or didn't say can't be verified in full as long as he is held in residential surveillance", Michael Caster, a spokesman for China Action, said on Twitter.

Dahlin's case followed the disappearance of two Hong Kong booksellers with European citizenship -- one from the former British colony and the other from Thailand. They reappeared in China, raising fears of Chinese authorities extending their reach internationally.

Lee Bo, who has a British passport, and Swede Gui Minhai were both born in China and were rumoured to be preparing a tell-all book about the love life of President Xi Jinping.

On Monday CCTV broadcast a video of Gui confessing to a drink-driving charge and saying he did not want the Swedish government to interfere with his case.

The Hong Kong government said Tuesday that its police had received information from Chinese authorities that Lee, who disappeared in December, was in China.