President Obama on Sunday joined other world leaders in expressing heightened concern about the security of Syria's chemical weapons amid increasing violence as rebel forces attempt to overthrow President Bashar al-Assad's regime.

The concern arose as the Assad regime threatened to use chemical and biological weapons if attacked by a foreign country, and a report that more than 100 people were killed Sunday as rebel fighters attempted to take control of border crossings and capture Syria's largest city, Aleppo.

The deaths, which included at least 24 government troops, bring the number killed to 19,000 since the unrest started this spring.

"Given the escalation in violence in Syria and the regime's increasing attacks on its own people, we remain very concerned about these weapons," White House press secretary Jay Carney said.

Carney also said Obama met Sunday morning with his senior national