Russia deeply regrets and condemns the U.S. strikes on Iranian nuclear sites, the Kremlin said on Monday.
The U.S. actions had increased the number of participants in the conflict and ushered in a new spiral of escalation, Kremlin spokesman Dmitry Peskov told reporters.
He added it was not yet clear what had happened to Iran's nuclear facilities and whether there was a radiation hazard.
The situation on the ground in Iran after the strikes cannot fail to be a cause of concern, he said.
Russia signed a strategic partnership treaty with Iran in January, although it did not include a mutual defence clause. Before Saturday's U.S. strikes, Moscow had warned that U.S. military intervention could destabilise the entire region and plunge it into the "abyss".
Peskov said U.S. President Donald Trump had not told Russian President Vladimir Putin in detail about the planned strikes in advanced, although they had discussed the possibility of U.S. military involvement more generally.
Asked what Russia was ready to do now, Peskov said Moscow had offered its services as a mediator, and what happened next would depend on what Iran needed.
Putin is due to meet Iranian Foreign Minister Abbas Araqchi later on Monday.
