The Pentagon decided to raise the alert at all its military bases and installations in the US, to “Force Protection Bravo” level, as defence against “homegrown extremists” inspired or instigated by Islamic State (IS) jihadists.

The threat level was raised by the Northern Command on Friday due to the “increased and predictable threat of terrorism” at all military bases.

The force protection alert has another two levels higher than Bravo, dubbed Charlie and Delta, the latter referring to a “specific” risk of a terrorist attack against military or civilian personnel of the defence department.

At the same time, force protection levels are different and less serious than those of DEFCON, which has a minimum of five that refer to specific military measures for dealing with threats to national security.

The new threat level is a measure of prudence, not based on any specific risk. It means stepping up the guard at US military bases, where hundreds of thousands of people in uniform and their families live and where civilian employees also work.

The new force protection level will entail a stricter checking of IDs, random security inspections and greater vigilance.

Recent attacks planned by Americans inspired by the Islamic State have influenced the decision.

Last weekend in Texas, two armed men opened fire as they burst into a cartoon exhibition of the Prophet Muhammad and were in turn shot and killed by police.

The IS claimed responsibility for the attack, though it remains uncertain whether they coordinated the shootout or inspired it.