U.S. supermarket chain Kroger Co. said on Tuesday it has started using unmanned autonomous vehicles to deliver groceries Scottsdale, Arizona in partnership with Silicon Valley startup Nuro.

The delivery service follows a pilot program started by the companies in Scottsdale in August and involved Nuro's R1, a custom unmanned vehicle.

The R1 uses public roads and has no driver and is used to only transport goods.

Kroger's deal with Nuro underscores the stiff competition in the U.S. grocery delivery market with supermarket chains angling for a bigger share of consumer spending.

Peers Walmart Inc and Amazon.com Inc have also invested heavily in their delivery operations by expanding their offerings and shortening delivery times.

Walmart, Ford Motor Co and delivery service Postmates Inc said last month they would collaborate to deliver groceries and other goods to Walmart customers and that could someday use autonomous vehicles.

Kroger (KR.N) said the service would be available in Scottsdale at its unit Fry's Food Stores for $5.95 with no minimum order requirement for same-day or next-day deliveries.