The U.S. Embassy in Ukraine said that attacking a nuclear power plant is a war crime, after Russia on Friday seized a Ukrainian nuclear facility that is the biggest in Europe.
The statement on the embassy's Twitter account went further than any U.S. characterization of Russia's actions in Ukraine since it launched its invasion Feb. 24.
"It is a war crime to attack a nuclear power plant. Putin's shelling of Europe's largest nuclear plant takes his reign of terror one step further," U.S. Embassy Kyiv said in its post.
Russian invasion forces seized Europe's biggest nuclear power plant in heavy fighting in southeastern Ukraine, triggering global alarm, but a blaze in a training building was extinguished and officials said the facility was now safe.
Russia's defense ministry blamed a fire at the plant on a "monstrous attack" by Ukrainian saboteurs and said its forces were in control.
The State Department sent a message to all U.S. embassies in Europe telling them not to retweet the Kyiv Embassy's tweet calling the attack a war crime, according to CNN, which said it reviewed the message.
The State Department did not respond to a request for comment from Reuters asking if the Kyiv Embassy's tweet reflects the position of the entire U.S. government.
Rights groups have alleged violations of international war crimes law in Ukraine, including the targeting of civilians, as well as indiscriminate attacks on schools and hospitals.
On Wednesday, U.S. President Joe Biden stopped short of calling Russia's actions war crimes, saying, "It's too early to say that."
Pentagon spokesperson John Kirby on Friday declined to answer the question, saying he would leave that determination to the International Criminal Court.
"This just underscores how reckless the Russian invasion has been and how indiscriminate their targeting seems to be. It just raises the level of potential catastrophe to a level that nobody wants to see," Kirby said in an interview with CNN.
"It is certainly not the behavior of a responsible nuclear power."
Britain has publicly accused Russian President Vladimir Putin's government of war crimes.
The ICC, the world's top war crimes prosecutor, on the request of 39 member states, is investigating reports of cluster bombs and artillery strikes on Ukrainian cities.
Karim Khan, a British lawyer named as the chief prosecutor of the ICC last year, said the crisis in Ukraine is a chance to demonstrate that those committing war crimes would be held to account.
Intentionally targeting civilians and civilian objects is a war crime, a U.S. State Department spokesperson told Reuters, adding that it is backing the investigation, particularly Khan's efforts to preserve evidence of possible atrocity crimes.
Kremlin spokesperson Dmitry Peskov has strongly denied claims that Russian forces have struck civilian infrastructure targets or residential complexes.