Europe's largest nuclear power plant has lost external electricity for the second time in five days, the head of the United Nations' nuclear watchdog said Wednesday.

International Atomic Energy Agency Director-General Rafael Mariano Grossi said he was informed of the situation at the Zaporizhzhia nuclear power plant in southeastern Ukraine on Wednesday morning. The Russian-occupied plant's "back-up diesel generators are now providing electricity for its nuclear safety and security functions," according to Grossi.

“This repeated loss of #ZNPP’s off-site power is a deeply worrying development and it underlines the urgent need for a nuclear safety & security protection zone around the site," Grossi said in a post on Twitter.

On Tuesday, Ukraine's state nuclear energy company Energoatom alleged that Russian forces have kidnapped Valery Martynyuk, the deputy head of the Zaporizhzhia nuclear power plant, and are detaining him in an unknown location. In a statement to ABC News, Energoatom accused Russian troops of trying to get information about the personal affairs of Zaporizhzhia employees in order to force Ukrainian personnel to work at Russia's state nuclear agency Rosatom as soon as possible.

Russian authorities announced Wednesday that eight people have been arrested in connection with Saturday's explosion on a key bridge linking Russia to the Moscow-annexed Crimean Peninsula.

Five of those detained are Russian citizens, while the others are Ukrainian and Armenian, according to
the Federal Security Service of Russia, the country's principal security agency.

Russia's state-run TASS news agency reported that the explosives used on Saturday were hidden in rolls of camera film.

Russian President Vladimir Putin has ordered a commission to investigate the blast. Moscow had been using Kerch 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. The Ukrainian Ministry of Defense said the bridge had "gone down," and images posted by official Ukrainian accounts on social media appeared to show the structure partially destroyed.

Putin responded to the bridge explosion by ordering missile attacks across Ukraine on Monday, striking civilian and critical infrastructure targets in major cities including Kyiv and Lviv. At least 19 people were killed and 105 others were injured, according to the Ukrainian State Emergency Service.

Seven people were killed and seven others were injured after Russian forces shelled the Zaporizhzhia region on Tuesday with M270 multiple launch rocket systems and artillery, the deputy head of President Volodymyr Zelenskyy's office, Kyrylo Tymoshenko, confirmed to ABC News.

-ABC News' Will Gretsky

NATO leaders warned Russia that it would meet attacks on allies' critical infrastructure with a "united and determined response."

In a news conference Tuesday, NATO Secretary-General Jens Stoltenberg said that while the defense alliance had not seen any changes in Russia's nuclear posture, it was vigilant and would proceed with a nuclear preparedness exercise of its own next week.

"Now is the right time to be firm and to be clear that NATO is there to protect and defend all allies. ... It would send a very wrong signal if we suddenly now canceled a routine, long-time-planned exercise because of the war in Ukraine," Stoltenberg said.

In next week's annual "Steadfast Noon" drill, NATO air forces will practice the use of U.S. nuclear bombs based in Europe with training flights, without live weapons.

Stoltenberg also pledged to boost the protection of critical infrastructure in response to the attack on the Nordstream gas pipelines, saying NATO had already doubled its presence in the Baltic and the North seas to over 30 ships supported by aircraft and undersea activities.

"We will further increase protection of critical infrastructure in light of the sabotage of the Nordstream pipelines," he said.

-ABC News' Will Gretsky