SEATTLE — A computer outage affecting sales registers disrupted sales Friday at 8,000 company-operated Starbucks stores in the United States and Canada. The outage affected 7,000 U.S. stores and 1,000 in Canada, company spokesman Jim Olson said Friday evening.

Customers found some stores closed and others offering free coffee. In Phoenix, some stores blocked off the entrance to their drive-thrus.

“We apologize to our customers for any inconvenience or confusion,” the company said in a statement.

Starbucks said it was working to resolve the outage. The exact nature of the problem wasn’t immediately made clear.

At a Starbucks store in Seattle’s South Lake Union neighborhood, customers were told staff members couldn’t process orders. They were, however, being offered coffee at no charge.

At a Starbucks in Phoenix, customers drove away angrily after seeing closed signs.

“I have a sleeping baby in the back and I’m waiting for a prescription,” said Claudia Larson, 40, of Scottsdale. “I wanted a coffee! I’m bummed!”