PORTLAND — A pizza chain says the credit card numbers of 900 customers were potentially exposed in a data breach at two pizza restaurants in Portland.

Otto Pizza said Friday that it experienced a data breach starting in May at two restaurants on Congress Street. Six other restaurants in Maine and Massachusetts weren’t impacted.

The company says no fraud has been reported, but it’s encouraging customers to contact their bank or credit card company if they see suspicious transactions. It says customers may want to take other steps to ensure they’re protected.

The company says it’s likely that fewer than 3 percent of the transactions at the stores through Aug. 13 were impacted. The company says it has taken steps to improve security.