CRIMEA - OCTOBER 8: A bridge connecting Crimea to Russia caught fire early Saturday morning causing suspension of traffic.

Footage and images, widely shared on social media, show that a road section collapsed and train carriages were burning on the bridge, which is the longest in Europe.

A huge explosion was heard before the massive fire broke out, according to eye-witnesses and video footage.

Russian President Vladimir Putin has ordered the formation of a state commission to investigate the incident, official news agency TASS cited Kremlin Spokesman Dmitry Peskov as saying.

Oleg Kryuchkov, an adviser to Russian-annexed Crimea’s leadership said on Telegram that a fuel storage tank has caught fire, according to preliminary reports.

"Navigable arches were not damaged. It is too early to speak about causes and consequences. Work to extinguish the blaze is underway," he added.

Ukraine's senior presidential adviser Mykhailo Podolyak said what happened on the bridge is just "the beginning."

"Everything illegal must be destroyed, everything stolen must be returned to Ukraine, everything occupied by Russia must be expelled," Podolyak added in a tweet.

The Kerch Bridge -- a pair of parallel bridges, one road and one railroad -- is a key supply route for Russia which was built after Crimea's annexation in 2014.

AA