Parts of a US city have been evacuated after a train carrying explosive chemicals derailed and blew up – sending a fireball and thick plumes of smoke towering into the sky.

The 47-carriage-long freight train came off the tracks in Sibley, Iowa, on Sunday, prompting the evacuation of nearly 100 people who live nearby.

The Union Pacific freight train derailed when a small bridge it was traveling over partially collapsed, causing the train to split in two.

As the train crashed the carriages towards the rear hit one and other and sparked a huge chemical explosion. The train was carrying a highly-flammable fertilizer substance and ammonium nitrate, Sibley Fire chief Ken Huls said. As the chemicals caught fire a large blaze broke out, sending clouds of black smoke in to the air.
Authorities told anyone within a five mile radius of the accident to leave their homes until emergency services get the fire under control.

Fire departments from the neighboring towns of Ashton, Little Rock, Melvin, Ocheyedan, and Sheldon were all sent to the scene to fight the blaze, which authorities said was ignited by diesel fuel.

The cause of the crash is under investigation, officials said. Hazmat teams were also called in to assist with the fire, which happened at around 2pm on Sunday.   Robynn Tysver, a Union Pacific spokesperson said that none of the train crew had been injured.