Chinese authorities are holding a Swedish citizen on suspicion of endangering national security, Beijing confirmed Wednesday, as it appeared he had been caught up in a crackdown on human rights lawyers.

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

Peter Dahlin was detained more than a week ago on his way to Beijing's international airport, the Chinese Urgent Action Working Group which he works for said in a statement.

Authorities had prevented embassy officials from speaking to him, it added, and his Chinese girlfriend had also disappeared.

China's foreign ministry spokesman Hong Lei said that a "Swedish citizen, Peter, is subject to criminal law enforcement in Beijing on suspicion of endangering China's national security".

Dahlin's organisation, also known as China Action, said it supports "barefoot" lawyers who provide pro-bono legal aid to grassroots victims of rights violations, from demolition and eviction to arbitrary detention.

"Peter has been arbitrarily detained on spurious accusations," it said, referring to a "six-month long assault on the country's human rights lawyers".

Under President Xi Jinping the ruling Communist Party has stepped up a campaign against outspoken academics, lawyers and human rights activists, which has seen hundreds detained and dozens jailed.

China this week confirmed the formal charging of at least nine human rights lawyers with "subversion" offences.

Detention of foreigners on national security grounds is rare in China, though Japan said four of its citizens were held last year and accused of spying.

US businesswoman Sandy Phan-Gillis has been held for six months in China over alleged espionage, supporters said in September.

A Canadian Christian couple who ran a coffee shop in the Chinese border city of Dandong were detained on espionage charges in 2014. The wife was later granted bail.

- 'No direct contact' -

China passed a new "national security" law in July that was criticised by rights groups for the vague wording of its references to "security", which raised fears it could give police wide-ranging discretionary powers over civil society.

Beijing has also drafted a new law that would put overseas non-governmental organisations under close supervision by Chinese police while operating in the country.

Chinese state-run media often accuse foreign NGOs of undermining national security and trying to foment "colour revolution" against the Communist Party.

Beijing is preventing embassy officials from speaking to Dahlin, China Action said, adding that he suffers from a potentially life threatening illness and his girlfriend -- a Chinese national -- is also missing.

"Despite constant requests by the Swedish Embassy, the Chinese authorities have denied direct contact with Peter and have not provided any communications from Peter to the embassy," the statement sent to AFP said.

"Peter suffers from Addison's Disease, a rare defect of the adrenal gland, which is potentially life threatening unless properly medicated daily."

Sweden's embassy in Beijing wrote in an email that it had "requested to meet the detained citizen", who it earlier said was a man in his mid 30s.

China will "facilitate Swedish consular officials' performance of their duty", foreign ministry spokesman Hong said.