Russia says a vital link to Crimea has partially reopened after an explosion and fire. President Vladimir Putin has ordered Russia's security service to tighten security for gas and power links to Crimea.

Russia's National Anti-Terrorism Committee said on Saturday that a truck exploded on the only road bridge linking Crimea and southern Russia. Two sections of the road bridge collapsed.

The blast also caused fuel tanks on a train to explode on a parallel railway bridge. The Russian Investigative Committee said three people were killed.

Putin instructed his government to form a commission to investigate the incident.

Putin also signed a decree vesting the Federal Security Service with authority to tighten security for the energy bridge and gas pipeline connecting Russia and Crimea.

The bridge to Crimea has served as a strategic supply route from Russia. Damage to the link is expected to hamper the activities of Russian forces.

Russia's government says the crossings have partially reopened to vehicles and trains. The country's defense ministry says Russian forces deployed in southern Ukraine are being supplied via land and sea routes.

Ukrainian officials have not said whether their country was involved in the explosion. But earlier this month, one of Putin's closest aides warned at a meeting in Crimea that there was a heightened risk of sabotage or terrorism.

Meanwhile, Russia's defense ministry said on Saturday that Army General Sergey Surovikin has been appointed as the new commander of Russian forces in Ukraine.

The move is apparently aimed at reversing the recent losses Russia has suffered after Ukraine's fierce counter-offensives in the country's east and south.