This video grab shows thick black smoke rising from a fire on the Kerch bridge that links Crimea to Russia. AFP
Russia said the blast set ablaze seven oil tankers by transported by train and collapsed two car lanes of the giant road and rail structure.
Dramatic social media footage showed the bridge on fire with parts plunging into the sea.
"Today at 6:07 am (0307 GMT) on the road traffic side of the Crimean bridge ... a car bomb exploded, setting fire to seven oil tankers being carried by rail to Crimea," Russian news agencies cited the national anti-terrorism committee as saying.
The bridge, personally inaugurated by President Vladimir Putin in 2018, is a vital transport link for carrying military equipment to Russian soldiers fighting in Ukraine.
Moscow had maintained the bridge crossing was safe despite the fighting.
The Kremlin spokesman said Putin had ordered a commission to be set up to look into the blast, Russian news agencies reported.
Russia's powerful investigative committee opened a criminal probe into the explosion and sent detectives to the scene.
A helicopter drops water to extinguish fuel tanks ablaze in the Kerch Strait, Crimea, on Saturday. Reuters
It said a truck exploded "on the automobile part of the Crimean bridge from the side of the Taman Peninsula".
This "caused seven fuel tanks to ignite on a train heading towards the Crimea Peninsula. As a result, two lanes partially collapsed."
While officials in Moscow stopped short of blaming Kyiv, an official in Russian-installed Crimea pointed the finger at "Ukrainian vandals".
Mykhailo Podolyak, adviser to the head of the office of Ukraine's presidency, took to Twitter posting a picture of a long section of the bridge half-submerged in the waters.
In Crimea, which Russia annexed from Ukraine in 2014, Russian authorities said local air defenses shot down a drone above the headquarters of the Russian Black Sea Fleet in Sevastopol.
The Kremlin announced on the same day the appointment of a new general to lead its Ukraine offensive following a series of battlefield setbacks that triggered unprecedented criticism of its army at home.
"Ships of the Black Sea Fleet repelled a drone attack in the waters of the Sevastopol Bay," the governor of Sevastopol, Mikhail Razvozhaev, said on Telegram. "Nothing has been hit in the city. We remain calm. The situation is under control."
Governor Dmytro Zhyvytskyy posted photographs of the charred shell of a four-story building and rescuers searching rubble. In a later Facebook post, he said many Russian soldiers and some local residents also were killed during the fighting on Sunday.
On both sides of the border, residents jolted out of sleep by the pre-dawn quake rushed outside on a cold, rainy and snowy winter night, as buildings were flattened and strong aftershocks continued.
Karnataka Police have arrested a woman from Kerala, who threatened to explode a bomb at the Bengaluru International Airport while arguing with the authorities regarding delay to board the flight, police said on Monday.
Syrian toddler Raghad Ismail was rushed to safety from the rubble of her home after it collapsed in a huge earthquake that has wreaked devastation in Syria and Turkey. But most of her family, including her mother, did not make it out alive.