WASHINGTON, March 6 (Reuters) - American Express Co said on Sunday it was suspending all operations in Russia and Belarus, joining a growing number of Western corporations to boycott business there over Moscow's invasion of Ukraine.

"In light of Russia's ongoing, unjustified attack on the people of Ukraine, American Express is suspending all operations in Russia," the credit card company said in a statement on its website.

"We are also terminating all business operations in Belarus," American Express said.

American Express follows U.S. counterparts Visa Inc and Mastercard Inc, which announced a suspension of their Russian operations the previous day, as well as payments company PayPal Holdings Inc.

American Express said its globally issued cards would no longer work in Russia at merchants or at ATMs. In addition, cards issued by Russian banks in Russia would no longer work outside the country on American Express' network.

The company said it has already suspended its relationships with Russian banks affected by U.S. and international sanctions. (Reporting by Chris Gallagher Editing by Chris Reese)