Transport Canada says it has confirmed a spate of serious bomb threats against major centres in the United States that have prompted some evacuations.

In the past month, four separate incidents have been reported in Colorado, Colorado Springs, Denver, and Albuquerque.

Transport Canada said the incidents are the most recent to affect major centres.

The department said the safety of employees and passengers remains at the forefront of its work.

The latest threats follow an incident at a transit centre in Colorado Springs earlier this month, when a bomb threat was made to the facility and the evacuation order was issued.

The suspect was arrested after police responded to a call for a bomb.

The threats have raised concerns that the threat environment has worsened in recent months.

In January, three people were killed and eight injured in a shooting at a downtown Calgary shopping centre.

The gunman was also killed.