WASHINGTON (AP) — Some hungry customers in the nation's capital may be surprised to discover a robot is delivering their pastrami on rye.

The delivery service Postmates is using knee-high, six-wheeled robots to bring food to customers. The robots can travel up to 4 mph and use sensors and cameras to navigate sidewalks and cross the street. Initially, the robots will be accompanied by a person, but eventually their progress will be monitored remotely.

Postmates says it's the first service of its kind in the United States.

Customers put their orders in online and once the robot arrives, they get a text with a code to unlock it and grab their food.

Postmates already uses human couriers to deliver food and goods in scores of cities. The robots are made by Starship Technologies.