The magnitude-6.1 temblor was the largest of more than 40 significant aftershocks that have followed the Jan. 12 quake. The extent of additional damage or injuries was not immediately clear.

Wails of terror rose from frightened survivors as the earth shuddered at 6:03 a.m. U.S. soldiers and tent city refugees alike raced for open ground, and clouds of dust rose in the capital.

The U.S. Geological Survey said Wednesday's quake was centered about 35 miles (60 kilometers) northwest of Port-au-Prince and 6.2 miles (9.9 kilometers) below the surface a little further from the capital than last week's epicenter was.

"It kind of felt like standing on a board on top of a ball," said U.S. Army Staff Sgt. Steven Payne. The 27-year-old from Jolo, West Virginia was preparing to hand out food to refugees in a tent camp of 25,000 quake victims when the aftershock hit.

Last week's magnitude-7 quake killed an estimated 200,000 people in Haiti, left 250,000 injured and made 1.5 million homeless, according to the European Union Commission.