Washington Post joins list of hacked US media

The Post said in a front page story that the attack was detected in 2011. It said Post company officials would not comment on the circumstances, duration of the intrusion or apparent origin of the online attack.

The paper quoted Post spokeswoman Kris Coratti as saying the paper worked with a security company to detect, investigate and resolve the situation "promptly" at the end of 2011.

"We have a number of security measures in place to guard against cyberattacks on an ongoing basis," Coratti was quoted as saying.

The attack coincided with the revelation of several high-profile security breaches.

The New York Times and The Wall Street Journal said earlier this week that they had been hacked, and pointed to attackers from China.

The two dailies accused Chinese hackers of targeting their computers in an apparent effort to spy on journalists covering China.

Meanwhile, Twitter said Friday that it too had been hammered by a sophisticated cyber attack similar to those that recently hit by the media outlets.

The popular microblogging site said the passwords of about 250,000 users were stolen, but did not confirm the source of the intrusion.

"This attack was not the work of amateurs, and we do not believe it was an isolated incident," Twitter information security director Bob Lord said in a blog post.

And the New York Times reported this week that the Bloomberg news agency was attacked by Chinese hackers after it published an article last June about wealth accumulated by relatives of Beijing's then-vice president Xi Jinping, who is expected to become China's president in March.

Twitter noticed an "uptick in large-scale security attacks aimed at US technology and media companies" Lord said, describing how the company detected attempts this week to get unauthorized access to data in the firm's network.

He noted that "the attackers were extremely sophisticated, and we believe other companies and organizations have also been recently similarly attacked."

He added that the cyber attackers may have gotten usernames, email addresses, passwords and other data.

As a precaution, Twitter invalidated passwords of accounts at issue and sent people email messages telling them to create new ones.

Twitter announced in December that the number of active users of the service had topped 200 million, in a sign of soaring growth.

Hillary Clinton said on Thursday as she wrapped up her tenure as US secretary of state that there has been an increase in hacking attacks on both state institutions and private companies.