Concern is raising over two Swedes who, independently of each other, have been detained in China on unclear accusations.

Peter Dahlin, a Swedish human rights worker, who was detained in China last week was suspected of acts detrimental to the country’s national security, the Ministry of Foreign Affairs said. It the country’s first comments on the case against the foreign human rights and legal reform advocate.

The Swedish Foreign Ministry said it had summoned China’s ambassador over the case on January 8, Reuters reports.

“We take a serious view on the fact that the embassy has not yet been allowed to visit the Swedish citizen detained in China,” Foreign Ministry spokeswoman Anna Ekberg said.

At the same time, the mysterious disappearance of five Hong Kong booksellers – including one Swede – is increasing.

The first to disappear was Gui Minhai, a Swedish citizen and co-owner of a publishing house specialising in books highly critical of China’s Communist Party. He vanished from his beachfront apartment in Thailand in October and is believed to have been taken to Mainland China by Chinese security agents. Publisher Lee Bo was the fifth person linked to Hong Kong bookseller Mighty Currents to go missing late December. He is a British passport holder.

In a tweet last week, Sweden’s former Prime Minister Carl Bildt asked rhetorically if China has started kidnapping people – including Swedish citizens – in the former British colony.

“Is China starting to kidnap individuals in Hong Kong as well as other countries? Including a Swedish citizen,” the Swedish politician wrote, linking to a Washington Post article about the mysterious events.

Sweden’s foreign ministry has previously said its embassies in Bangkok and Beijing were investigating reports that one of its nationals had been detained in Thailand or China.