WASHINGTON (Reuters) - Social media operators such as Twitter, Facebook and YouTube would have to notify federal authorities of online "terrorist activity," according to the text of a bill approved by the Senate Intelligence Committee and seen by Reuters on Wednesday.

The types of communication include postings related to "explosives, destructive devices, and weapons of mass destruction," according to the text. An official familiar with the bill said it was sent to the Senate floor for a vote.

The official said its main purpose was to give social media companies additional legal protection if they reported to the authorities on traffic circulated by their users, rather than coerce them to spy on users.

It was unclear when the Senate might vote on the bill.

A congressional official said it was also unclear whether the House of Representatives would pursue similar legislation, which would be necessary for the proposed requirement to become law.

Social media groups have been widely used by militant groups such as Islamic State and Yemen-based Al Qaeda in the Arabian Peninsula to recruit members and circulate bomb-making instructions.

At a Senate Judiciary Committee hearing on Wednesday, Dianne Feinstein, the senior Democrat on the Senate Intelligence Committee and sponsor of the legislation, said social media companies should be working with the government to prevent the use of their systems by violent militants.

"Twitter, FB and YouTube all, as I understand it, remove content on their sites that come to their attention if it violates their terms of service, including terrorism," Feinstein said.

But, she said, "the companies do not proactively monitor their sites to identify such content nor do they inform the FBI when they identify or remove their content. I believe they should."

The social media legislation, part of a larger intelligence authorization bill, would not require social media companies to monitor specific users or content posted by individuals. Nor would it penalize companies that failed to comply.

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"We share the government's goal of keeping terrorist content off our site," Facebook's Head of Policy Management Monika Bickert said in a statement.

"Our policies on this are crystal clear: we do not permit terrorist groups to use Facebook, and people are not allowed to promote or support these groups on Facebook. We remove this terrorist content as soon as we become aware of it.”

A representative of Twitter said that her company had not taken a position on the legislation. Google did not immediately respond to requests for comment.

(Reporting by Mark Hosenball; additional reporting by Lindsay Dunsmuir; Editing by Howard Goller, Eric Walsh and Ken Wills)