WASHINGTON (Reuters) - The United States is increasing security at government buildings in Washington and other cities because of continuing terrorist threats and last week’s attack on the Canadian parliament, the Homeland Security Department said on Tuesday.

U.S. Homeland Security Secretary Jeh Johnson takes part in a question and answer session during a Canadian-American Business Council luncheon in Ottawa September 30, 2014. REUTERS/Chris Wattie

A U.S. official said there had been no credible information about any specific plot against a U.S. target, but many calls to attack the United States have been issued, including on social media, by supporters of groups such as Islamic State and al Qaeda.

The official, who spoke on condition of anonymity, said the new measures followed a stream of threats from militant groups which U.S. agencies have been monitoring for months.

Homeland Security Secretary Jeh Johnson said in a statement: “Given world events, prudence dictates a heightened vigilance in the protection of U.S. government installations and our personnel.”

He said: “The reasons for this action are self-evident: the continued public calls by terrorist organizations for attacks on the homeland and elsewhere, including against law enforcement and other government officials, and the acts of violence targeted at government personnel and installations in Canada and elsewhere recently.”

Johnson said he had ordered extra security at “various U.S. Government buildings in Washington DC and other major cities and locations around the country.”

Details of the actions to be taken by the Federal Protective Service, which covers more than 9,500 facilities, were “law-enforcement sensitive” and would vary in different locations. Johnson gave no details.

Johnson warned U.S. state and local security services to be on their guard especially for possible “small-scale attacks by a lone offender or a small group of individuals.”

Last Wednesday, Michael Zehaf-Bibeau, a man described by Canadian authorities as troubled and drug addicted, shot dead a soldier at Canada’s war memorial in Ottawa and charged into Parliament, exchanging fire with security officers before being shot dead.

Police said Zehaf-Bibeau, a convert to Islam, made a video of himself before the attack, which they said provided evidence that he had ideological and political motives.