SAN FRANCISCO -- Store clerks in three San Francisco neighborhoods were robbed at gunpoint over a four-hour period early this morning, San Francisco police said.

Police do not yet know whether the robberies are connected.

The first happened in the city's Mission District at about 1:20 a.m. when two men robbed a store clerk in the 3100 block of 24th Street before fleeing on foot, police said.

At 4:56 a.m., a second store clerk was robbed at gunpoint by two men in the 3000 block of San Jose Avenue in the city's Mission Terrace neighborhood.

About 10 minutes later, a third clerk was robbed at gunpoint by three men in the 2000 block of Ocean Avenue, in the city's Ingleside Terrace neighborhood.

None of the clerks was injured and no arrests have been made, police said.