The FBI is "concerned" by reports that secret "police stations" linked to China have been set up across the US.

A September report issued by the NGO Safeguard Defenders revealed the presence of these stations around the world, including in New York.

The FBI's director, Christopher Wray, told senior politicians that the agency was monitoring reports of such centres across the country.

"We are aware of the existence of these stations," Mr Wray said.

"To me, it is outrageous to think that the Chinese police would attempt to set up shop, you know, in New York, let's say, without proper coordination," he added. "It violates sovereignty and circumvents standard judicial and law enforcement cooperation processes."

Asked if the stations violated US law, Mr Wray said the FBI was "looking into the legal parameters".

The senior intelligence official was speaking at a US Senate Homeland Security and Governmental Affairs Committee hearing, where he was grilled by senior lawmakers.

According to the Spain-based NGO Safeguard Defenders, Chinese public security bureaus established the "overseas police service stations" in several continents, including two in London and one in Glasgow. In North America, it found stations in Toronto and in New York.

The units were reportedly created to tackle transnational crime and to provide administrative services to Chinese nationals abroad, such as renewing drivers' licences abroad and other consular services.

However, Safeguard Defenders said they also serve a "more sinister goal", by contributing to "cracking down on all kinds of illegal and criminal activities involving overseas Chinese".

China has denied running the overseas stations.

Mr Wray said the United States had opened a number of charges related to the Chinese government harassing, stalking, monitoring and blackmailing people in the US who had been critical of Chinese President Xi Jinping.

"It's a real problem and something that we're talking with our foreign partners about, as well, because we're not the only country where this has occurred," he said.

In October, the US unsealed criminal charges against seven Chinese nationals accused of spying on and harassing a US resident and his family as part of efforts by the Chinese government to return one of them to China.

Last month, one of the Chinese "police stations" in the centre of the Irish capital Dublin was ordered to close by the government as a result of Safeguard Defenders' work.

And Canadian intelligence officials recently said they were investigating accusations that China had opened unofficial "police" stations on Canadian soil.