May 14 (Reuters) - People in four suburbs of the Canadian oil sands city of Fort McMurray in Alberta were ordered to evacuate on Tuesday due to an approaching wildfire, the local authority said.

The huge fire has gradually been moving closer to the city and is now about 13 km (8 miles) away to the south west.

In a statement, the authority said residents in the suburbs of Abasand, Beacon Hill, Prairie Creek and Grayling had to leave by 4 pm Mountain Time (2200 GMT).

"These neighborhoods directly interface with where the fire could potentially spread. Regional Emergency Services will better be able to defend these neighborhoods from wildfire if they are uninhabited and clear," it said.

The city's residents were told last week to be prepared to evacuate if need be.

In 2016, a huge wildfire in Fort McMurray forced the evacuation of 90,000 residents and shut in more than a million barrels per day of oil output. (Reporting by David Ljunggren and Nia Williams Editing by Chris Reese and Sandra Maler)