In this Tuesday, July 24, 2012 photo, a damaged vehicle from clashes between Free Syrian Army soldiers and Syrian government troops is seen at the border town of Azaz, some 20 miles north of Aleppo, Syria. Turkey sealed its border with Syria to trucks on Wednesday, July 25, 2012 cutting off a vital supply line to the embattled nation as fighting stretched into its fifth day in the commercial capital of Aleppo.WASHINGTON - The Obama administration is warning of a possible Syrian massacre in the city of Aleppo.
State Department spokeswoman Victoria Nuland says the U.S. has "grave concerns" about tanks and fighter jets being used in a densely populated city.
She says thousands of people are spilling out of Aleppo and calls the onslaught a "desperate" attempt by a government losing control of its country.
Rebels who've been fighting for six days in Syria's commercial capital of 3 million people are bracing themselves amid reports the government is massing reinforcements to retake the city. They are reporting artillery strikes and strafing by attack helicopters and fighter jets.
Nuland said: "The concern is that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for."