BEIRUT — Fierce clashes spread to new neighborhoods in Aleppo in a fourth day of fighting in Syria's commercial capital, showing the resilience of a rebel offensive on strongholds of President Bashar Assad's regime.

President Barack Obama also warned Syria against using its chemical weapons after Damascus acknowledged for the first time on Monday that they possessed such weapons and threatened to use them against any "foreign aggression."

Syria's rebels have recently seized the offensive in the country's bloody civil war, which activists say has killed at least 19,000 people since it began in March 2011. Rebel fighters over the past two weeks have taken the fight to the Assad regime in the country's two largest cities, killed four senior security officials in a bold bomb attack in the heart of the capital and captured a string of border crossings with Iraq and Turkey.