Twitter is warning some users that they may be the target of security breaches by "state-sponsored actors."

The microblogging service sent emails to an unknown number of users warning that hackers may be trying to obtain phone numbers, email addresses and IP addresses, according to Twitter users who have received the emails. The emails warn that while Twitter is not certain recipients' personal information was obtained, it has reason to believe they were intended targets.

"At this time, we have no evidence they obtained your account information, but we're actively investigating this matter," according to Coldhak, a Canada-based nonprofit organization that posted a copy of the letter it received on Twitter. "We wish we had more we could share, but we don't have any additional information we can provide at this time."

The emails go on to advise users to take precautions to safeguard their personal information, including offering a link to information on Tor, software that allows for anonymous communication on the Internet. Others who tweeted about having received the warning included security researchers and journalists.

A spokesman for San Francisco-based Twitter confirmed the authenticity of the emails but declined to provide further comment.

The warning, like similar ones issued previously by Facebook and Google, underscore the growing frequency of state-sponsored hacks, typically used to steal government information or intellectual property. China and the US have for years traded accusations of high-tech espionage, while a massive hack last year that crippled Sony Entertainment and revealed the inner workings of the studio was traced by the FBI back to hackers working for the North Korea government.

Social media in particular has become a popular battlefield for hackers. The Syrian Electronic Army, a hacking group that supports the regime of Syrian President Bashar al-Assad, has claimed responsibility for several hacks of news sites and company Web sites. The group has hacked into the Twitter account of the parody news site The Onion and it also has gotten into the accounts of the Associated Press , NPR, CBS, the Guardian, and the BBC.

Last month, the hacker collective Anonymous declared war on the Islamic militant group ISIS, saying it wants to steal information from that Web traffic and force the group into Internet obscurity. ISIS has used the Internet to recruit new members from around the world and to spread chilling images of executions and other violence.