President Vladimir Putin’s flagship bridge to Crimea was damaged in a blast that hit a fuel train and caused the partial collapse of the only road link running from the Russian mainland to the Black Sea peninsula it annexed in 2014.

A truck explosion on the span’s roadway caused seven fuel tanks on the train to ignite as it crossed the Kerch Strait Bridge early Saturday, Russia’s National Antiterrorism Committee said, according to the state-run Tass news service. The bridge is closed to traffic following the partial collapse of two sections of the road bridge running alongside the railway, it said.

It’s unclear so far what caused the explosion and there’s been no claim of responsibility. Video of the scene on social media showed the train on fire and a section of the road bridge collapsed into the sea.