Unknown hackers besieged Twitter this week and may have gained passwords and personal information of up to 250,000 users during the cyber-attack.
Twitter confirmed it was hacked in a blog post, saying that the passwords were encrypted and that it already reset them as a "precautionary measure."
The social network is "in the process" of notifying affected users.
Twitter noted recent revelations of cyber attacks against the New York Times and the Wall Street Journal, but did not elaborate on the origin or methodology of the attacks.
"This attack was not the work of amateurs, and we do not believe it was an isolated incident," Twitter said on its website, following the hacking.
"The attackers were extremely sophisticated, and we believe that other companies and organizations have also been recently similarly attacked."
Twitter, which has 200 million active monthly users, said it was working with government and federal law enforcement officials to track down the attackers.
The company did not specifically link the attacks to China in the blog post, in contrast to the Times and Journal, which both said the hackers originated in China.
Twitter, known for its 140-character messages, could not speculate on the origin of the attacks as its investigation was ongoing, said spokesman Jim Prosser.
"There is no evidence now that would indicate passwords were compromised," said Prosser.
Whoever was responsible and why, it's a trend that isn't going away anytime soon.
Last month, U.S. Department of Homeland Security Secretary Janet Napolitano warned against the imminent risk of a 9/11-level cyber attack.