The United States says it is concerned that Syrian forces are preparing to carry out a massacre in Aleppo, Syria's largest city.

State Department spokeswoman Victoria Nuland said Thursday there are columns of tanks outside the city that seem to be massing for an attack. She said Syrian forces already have hit the city with fighter jets and helicopter gunships, in a desperate effort to keep control of Aleppo.

Nuland said Aleppo is a densely populated civilian area and a world heritage site of beauty and renown. She said U.S. hearts are with the people of Aleppo.

The city had been relatively quiet until two weeks ago when rebels launched an operation to seize control.

The Syrian Observatory for Human Rights reported Thursday that shelling in several Aleppo neighborhoods killed a girl and wounded seven other people.