Starbucks is to install panic buttons for staff as it cracks down on customers who sit in its shops or try to use its lavatories without buying anything.

Starbucks reported a smaller-than-expected fall in first-quarter comparable sales on Tuesday, in early signs of the struggling coffee chain benefiting from CEO Brian Niccol's efforts to revive demand.

Starbucks on Tuesday reported better-than-expected sales in its fiscal first quarter as some of its turnaround efforts start to take hold. The Seattle coffee giant said its revenue was flat at $9.4 billion for the 13-week period ending Dec. 29. That beat Wall Street's forecast of $9.3 billion, according to analysts polled by FactSet.