As shelling intensified in Damascus, residents scrambled to pull people trapped under debris.

U.S. Secretary of State John Kerry said on Tuesday he had discussed Syria's use of chemical weapons with Russia and the international community's patience with Syrian President Bashar al-Assad's behavior was wearing thin.

Speaking to reporters via video conference from Boston, Kerry said he was confident Assad's government was responsible for a "preponderance" of chemical attacks against his own people and that "everyone's patience is wearing thin."

Kerry said it was one of the issues he discussed with Russian Foreign Minister Sergei Lavrov by phone on Monday and was confident Lavrov would raise it with Assad, who agreed in 2013 under a U.S.-Russia brokered deal to dismantle the country's chemical weapons arsenal.

He said chemical weapons were dropped from aircraft and the United States was putting together data to support its claims that Assad's government was responsible for attacks.