Russian President Vladimir Putin ordered a commission to investigate an explosion that damaged a key bridge linking Crimea and Russia. Russia had been using the bridge as a key supply route for bringing in troops and ammunition into southern Ukraine.

The Ukrainian Security Service declined to comment on rumors of its involvement in the bridge's explosion.

Putin also signed a decree instructing tighter security for the bridge and the infrastructure supplying electricity and natural gas to the peninsula.

The blast coincided with the naming of Air Force General Sergei Surovikin as the commander of all Russian troops in Ukraine.

-ABC News' Patrick Reevell and Jason Volack

Ukraine's Zaporizhzhya Nuclear Power Plane lost its last external power source due to renewed shelling, the International Atomic Energy Agency Director General Rafael Grossi said in a statement Saturday.

The plant is now relying on emergency diesel generators for the electricity it needs for reactor cooling and other essential nuclear safety and security functions, according to Grossi.

The plant's connection to the power line was cut at around 1 a.m. local time. Sixteen of the plant’s diesel generators started operating automatically, providing its six reactors with power. After the situation stabilized, 10 of the generators were switched off, according to Grossi.

"The resumption of shelling, hitting the plant’s sole source of external power, is tremendously irresponsible. The Zaporizhzhya Nuclear Power Plant must be protected," Director General Grossi said. "I will soon travel to the Russian Federation, and then return to Ukraine, to agree on a nuclear safety and security protection zone around the plant. This is an absolute and urgent imperative."

Three people were killed on Saturday in the explosion that collapsed portions of the bridge linking Russia to Crimea, a Russian official said.

The Russian Investigative Committee also said it had identified the driver of the truck that was allegedly blown up on the bridge.

Russia’s response should be tough, said Leonid Slutsky, chairman of the State Duma Committee on International Affairs.

"If the Ukrainian trace is confirmed in the state of emergency on the Crimean bridge, the consequences will be inevitable," Slutsky wrote on his Telegram channel on Saturday.

He said he has no doubt that "Kyiv is behind the organization of this attack." Ukrainian officials have not taken credit for the blast. Ukraine’s official government Twitter account tweeted the phrase “sick burn” after the explosion, but did not directly reference the blast.

"This is not just an emergency," Slutsky wrote. "It could be an act of state terrorism."

The railway infrastructure restoration has been started after the fire on the bridge was contained and extinguished, Crimean Railway said.

-ABC News' Somayeh Malekian and Tanya Stukalova

Russian officials said the explosion that damaged the key bridge linking Crimea and Russia came from a truck.

"Today at 6:07 a truck was blown up on the automobile part of the Crimean Bridge from the side of the Taman Peninsula," Russia's National Anti-Terrorism Committee posted online. "It resulted in the ignition of seven fuel tanks of the train, along the direction of the Crimean Peninsula. There was a partial collapse of two automobile spans of the bridge. The arch over the navigable part of the bridge was not damaged."

Russian investigators were at the scene, attempting to "establish the circumstances of the explosion," the committee said.

Russian supply lines into Crimea were likely to be disrupted by the blast. Crimean authorities said they would instead get supplies from Russia's newly annexed territories.

-ABC News' Somayeh Malekian and Tanya Stukalova