WASHINGTON (AFP) — Skype, the online text messaging and voice service, said Thursday it was "extremely concerned" by monitoring of Internet chat by its Chinese partner reported by Canadian researchers.

Citizen Lab, a group of computer security experts at the University of Toronto, revealed Wednesday that China was spying on Skype users, censoring politically sensitive messages and storing them on computer servers.

Skype said it learned just Wednesday that a previously disclosed text filter operated by TOM-Skype, a joint venture between Chinese mobile firm TOM Online and Skype, had been altered.

"Last night, we learned that this practice was changed without our knowledge or consent and we are extremely concerned," Skype, which is owned by US online auction house eBay, said.

"We deeply apologize for the breach of privacy relating to chat messages on TOM's servers in China and we are urgently addressing this situation with TOM," the company said.

In a 16-page report, Citizen Lab said "TOM-Skype is censoring and logging text chat messages that contain specific, sensitive keywords and may be engaged in more targeted surveillance.

"These logged messages contain keywords relating to sensitive topics such as Taiwan independence, the Falun Gong, and political opposition to the Communist Party of China," it said.

"These text messages, along with millions of records containing personal information, are stored on insecure publicly-accessible web servers," Citizen Lab added.

The researchers said they were able to gain access to the log files on eight TOM-Skype servers and work out exactly which keywords triggered the server to record a particular message.

They said the log files "contain the full content of the censored messages along with the IP address of the TOM-Skype user, the username that sent the message and the date and time the message was sent.

"With just one username it is possible to identify all the users that have sent messages to or received messages from the original user," they said.

"What is clear is that TOM-Skype is engaging in extensive surveillance with seemingly little regard for the security and privacy of Skype users," Citizen Lab said.

The report, "Breaching Trust: An analysis of surveillance and security practices on China's TOM-Skype platform," was published on Wednesday on the website of Information Warfare Monitor, a joint project between Citizen Lab and the SecDev Group, a think-tank on security issues based in Ottawa.

The monitoring of TOM-Skype is not the first time concerns have been raised about surveillance and censorship by the Chinese authorities and what has been described as the "Great Firewall of China."

At the World Economic Forum in Davos in January, Wang Jianzhou, the head of China Mobile Communications Corp. which has more than 300 million subscribers, revealed the mobile phone company had unlimited access to the personal data of its customers and handed it over to Chinese security officials when asked.

US Internet firm Yahoo has been embroiled in controversy after it provided information about a private email account to Chinese authorities that allowed them to arrest a dissident.

Yahoo defended its actions on the grounds that it had to comply with China's laws in order to operate there.

TOM Online, a subsidiary of the Hong Kong-based Tom Group Ltd., formed a joint venture with Skype in 2004.