US credit card and payments giant American Express said Sunday it is suspending its operations in Russia and Belarus over Moscow's "unjustified" attack on Ukraine, the latest financial services blow to Russia over its invasion.

"In light of Russia's ongoing, unjustified attack on the people of Ukraine, American Express is suspending all operations in Russia," the company said in a statement, noting that American Express cards will no longer work at merchants or ATM money machines in Russia.

"We are also terminating all business operations in Belarus," it added.

Earlier, the UN nuclear watchdog expressed "deep concern" over reports that communication from Europe's largest nuclear power plant seized by Russia in Ukraine has been disrupted.

Invading Russian forces attacked and seized the Zaporizhzhia nuclear power plant in Ukraine on Friday.

The International Atomic Energy Agency (IAEA) said in a statement that Ukraine informed it that the plant management is now under orders from the commander of the Russian forces.

Ukraine has also reported that the Russian forces have switched off some mobile networks and the internet, and that telephone lines, emails and fax were not functioning anymore.

They said mobile phone communication was still possible with poor quality, the IAEA said.

"I'm extremely concerned about these developments that were reported to me today," IAEA Director General Rafael Grossi said.

"In order to be able to operate the plant safely and securely, management and staff must be allowed to carry out their vital duties in stable conditions without undue external interference or pressure," he added.

He expressed "deep concern" over "the deteriorating situation regarding vital communications between the regulator and the Zaporizhzhia NPP (nuclear power plant)".

"Reliable communications between the regulator and the operator are a critical part of overall nuclear safety and security," he said.

On Sunday, Russian President Vladimir Putin assured French President Emmanuel Macron of the "physical and nuclear safety" of the Zaporizhzhia nuclear plant, according to the Kremlin.

The IAEA said Ukraine's nuclear regulator also reported that communicating with personnel at the Chernobyl nuclear power plant, which was taken by Russian forces on February 24, was only possible with emails at the moment.

Staff there are also reportedly unable to rotate -- leading to Grossi stressing "the importance of operating staff being able to rest to carry out their important jobs safely and securely".