WASHINGTON (REUTERS) - The US military has ordered security at its bases around the United States to the highest level in nearly four years, Army Colonel Steve Warren, a Pentagon spokesman, said on Friday.

He characterised the threat level as low to low-middle.

The heightened security level covered everything from recruiting stations to National Guard posts and military bases and camps in the continental United States, Alaska and US territory in the Caribbean.

Warren told reporters the heightened security was based not on any specific threats but a general climate that included last weekend's killing of two men who opened fire outside a Texas exhibit of cartoons of the Prophet Muhammad.

A US official speaking on condition of anonymity said the government had some specific information that people might be inspired by, or connected to, the two men and attempt an attack.

Social media postings indicate the Garland, Texas, gunmen had contact shortly before the attack with at least two militants, including a British man linked to Syria-based Islamic State rebels, officials said.

Investigators believe the Garland attackers principally radicalized themselves through Internet contacts, and were not directly ordered or encouraged to carry out the attack by Islamic State leaders.

Warren said the heightened security level would require more bag checks at US military bases and posts, possibly leading to longer queues and traffic backups in some places.

The decision to increase security at US bases was made by Admiral Bill Gortney, commander of the US Northern Command, responsible for troops in North America, Warren said. The decision did not affect bases outside his region.

Warren said the last time security was raised to an elevated level at bases across the United States was on Sept 11, 2011, the anniversary of the 2001 attacks on the Pentagon and the World Trade Center towers.