Syrian opposition activists say security forces are using using attack helicopters and machine guns to pound rebel targets in Damascus, the capital, and Aleppo, Syria's largest city.

The opposition says the Syrian army has moved thousands of troops to Aleppo and that fighting rages on the ground.

White House spokesman Jay Carney calls the reported use of attack helicopters and tanks in Aleppo a sign of the depths of depravity of President Bashar al-Assad.

Carney also said the United States has confirmed that two more Syrian diplomats — the ambassadors to Cyprus and the United Arab Emirates — have defected. He calls this another indication that Mr. Assad's days are numbered.

Also, Turkey has closed key border crossings to commercial traffic from Syria, but says they remain open for refugees.