US lawmakers expressed concern on Thursday over the monitoring of social networking sites such as Facebook and Twitter by the Department of Homeland Security.

Department officials defended the practice, meanwhile, at a congressional hearing, saying they monitor social media mostly for "situational awareness" about breaking news events and adhere to strict privacy guidelines.

Representative Patrick Meehan, the chairman of the House Homeland Security Subcommittee on Counterterrorism and Intelligence, said he backs "intelligence collection within the rules of law" but has free speech concerns.

"In my view, collecting, analyzing, and disseminating private citizens' comments could have a chilling effect on individual's privacy rights and people's freedom of speech and dissent against their government," Meehan said.

"I fully recognize that if an individual willingly uses Facebook, Twitter, or the comments section of a newspaper website, they in effect forfeit their right to an expectation of privacy," the Republican from Pennsylvania said.

"However, other private individuals reading your Facebook status updates is different than the Department of Homeland Security reading them, analyzing them and possibly disseminating and collecting them for future purposes," he said.

Representative Jackie Speier, a Democrat from California, said she was disturbed by monitoring of social media for reaction to government policies or programs and that it should not be a "political operation."

Richard Chavez, the director of Homeland Security's Office of Operations Coordination and Planning, told the committee that the monitoring program was not being used for that purpose.

"I am not aware of any information we have gathered on government proposals," he said.

The monitoring of social media by Homeland Security came to light following a lawsuit filed in December by the Electronic Privacy Information Center. It is being carried out under an $11 million contract with General Dynamics.

Mary Ellen Callahan, Homeland Security's chief privacy officer, told the committee that strict protections for privacy and civil liberties have been built into the program.

"If you can't do it offline, you can't do it online," Callahan said.

"We don't collect information on individuals," she said. "We do not monitor them.

"But individuals may be the first person at the scene," she said. "They may go and report there's been a train derailment in Michigan."

Chavez said the monitoring was of "keywords associated with events" such as natural disasters and potential security threats but not of individuals.

The Department of Homeland Security is not the only US government agency interested in mining social media.

The Federal Bureau of Investigation asked information technology contractors last month about the feasibility of building a similar monitoring tool.

The FBI said it is seeking an "open source and social media alert, mapping and analysis application solution" for its Strategic Information and Operations Center (SIOC).

"Social media has become a primary source of intelligence because it has become the premier first response to key events and the primal alert to possible developing situations," the January 19 request from the FBI said.

The FBI said the tool "must have the ability to rapidly assemble critical open source information and intelligence that will allow SIOC to quickly vet, identity, and geo-locate breaking events, incidents and emerging threats."