China has said the arrest of a journalist working for the newswire Bloomberg is an "internal affair", warning others not to interfere.

Chinese citizen Haze Fan was detained last week, accused of endangering national security by authorities.

It is the latest in a string of arrests or expulsions of journalists in China.

The European Union (EU) responded by urging China to release all journalists held in connection with their reporting.

In a statement released on Saturday, the EU said it expected Chinese authorities to grant Ms Fan "medical assistance if needed, prompt access to a lawyer of her choice, and contacts with her family".

The Foreign Correspondents Club in China (FCCC) also expressed its solidarity, adding that international media depended on its Chinese staff.

However, the Chinese Embassy in the EU responded on Sunday, saying Ms Fan was "suspected of engaging in criminal activities that endanger China's national security and was recently taken into compulsory measures by the Beijing State Security Bureau in accordance with the law".

On its official WeChat account, it added that the case is currently being investigated in accordance with the law and that Ms Fan's rights are fully ensured.

This is "entirely an internal affair of China, and no other country or organization has the right to interfere".

Ms Fan has been at Bloomberg since 2017, having previously worked for news agency Reuters, as well as for CNBC, Al Jazeera and CBS News.

She was seen being escorted from her apartment building by plain-clothes security officials on 7 December, shortly after she had been in contact with one of her editors.

"We are very concerned for her, and have been actively speaking to Chinese authorities to better understand the situation," a spokeswoman for New York-based Bloomberg said. "We are continuing to do everything we can to support her while we seek more information."

The FCCC said in a tweet it "expresses its solidarity with the talented Chinese nationals who perform an invaluable service for foreign media outlets in China".

"Chinese nationals offer critical research and linguistic support for foreign reporting in China. Without their support, it would be difficult for foreign media to operate in China.

The FCCC also said it was seeking "clarity on why authorities detained Haze Fan".

Ms Fan is not the first journalist to run into trouble with Chinese authorities this year.

Earlier in 2020, mainland China effectively expelled journalists from three leading US newspapers when it ordered reporters from the New York Times, the Washington Post and the Wall Street Journal to return their media passes within days as its relations with Washington worsened.

In August, authorities in Beijing detained China-born Australian citizen Cheng Lei, a journalist working for the Chinese state-run broadcaster CGTN, on national security grounds. In September, two Australian correspondents abruptly left China after they were questioned by China's state security ministry.

In Hong Kong, pro-democracy media tycoon Jimmy Lai earlier this month was charged under the territory's controversial new national security law.