Port-au-Prince, Haiti (CNN) -- A strong aftershock rocked Haiti on Wednesday morning just as much-needed medical aid was set to reach the earthquake-ravaged.

The 6.1-magnitude aftershock was about 6.2 miles deep, with an epicenter about 35 miles (60 kilometers) west-southwest of the capital Port-au-Prince, the U.S. Geological Survey said.

It rattled people struggling to recover from the devastating 7.0-magnitude earthquake that walloped the impoverished nation January 12, killing at least 72,000 people.