The US military bolstered security at bases across the country Friday after the FBI voiced concern that Islamist extremists could target troops or police officers.

The head of US Northern Command, Admiral William Gortney, ordered the heightened alert "as a prudent measure," Pentagon spokesman Colonel Steven Warren said.

The move, he said, was designed "to remind installation commanders at all levels within the NORTHCOM area of responsibility to ensure increased vigilance in safeguarding of all DoD (Defense Department) personnel and facilities."

This raised the official alert status one notch higher on a five-point scale, from the fourth level "Alpha" to the third level "Bravo," which the military describes as an "increased and predictable threat of terrorism."

Officials said most of the additional security measures would likely not be readily apparent to the public, apart from perhaps more bags being searched or ID cards being checked at base entrances.

The move came after two men attempted to storm an exhibition on Sunday showing caricatures of the Prophet Mohammed in a Dallas suburb. The two heavily-armed gunmen were shot dead by a local police officer before they could approach the building where the event was being held.

Pentagon chief Ashton Carter said on Thursday that the two assailants had been "inspired" -- but not directed -- by the Islamic State group to stage the failed attack.

"The decision to do this now is informed by a generally heightened threat level," Warren told reporters.

"We've seen what happened in Texas. We've seen other social media and Internet-based discussions and threats. We have detected a general increase in the overall (threat) environment."

- Online recruiting by IS -

FBI Director James Comey reportedly said on Thursday that authorities were concerned about the IS group encouraging attacks on "the uniformed military and law enforcement" via online propaganda.

There are "hundreds, maybe thousands" of people in the United States who had received recruitment messages from the jihadists, Comey said.

The risk posed by IS-inspired homegrown militants was a factor in the decision to raise the security level but not the only factor, defense officials said.

The FBI chief's comments echoed warnings from lawmakers and experts Thursday who said the IS group's social media efforts are carried out on a vast scale and at a tempo that Western governments are unable to keep up with.

The IS has 2,000 people tweeting for the group 150 times a day, according to J.M. Berger, an author and fellow at the Brookings Institution think tank.

The group's innovative and aggressive approach online has given it "an unprecedented level of success" compared to other extremist organizations, Berger, who has tracked IS activity in social media, told the Senate Homeland Security committee.