Syrian opposition activists report heavy gunfire in the northern city of Aleppo, where government forces have been battling rebels who are trying to seize control of the commercial hub.

Activists say fierce fighting is underway (Tuesday) in several neighborhoods, and 15 people were killed as security forces put down a prison mutiny in the city.

The opposition Local Coordination Committee said government forces also shelled areas outside of Damascus on Tuesday, as well as Deir Ezzor and Homs.

In the United States, President Barack Obama joined other world leaders in warning Syria against using chemical weapons. Mr. Obama said Monday that Mr. Assad would be held accountable if Syria used poison gas or other deadly chemicals.

His comments came after a Syrian Foreign Ministry spokesman said the government would never use chemical weapons against its own people, but would unleash them against what he called foreign invaders. He said the military is securely guarding the nation's weapons stockpile.

U.N. Secretary-General Ban Ki-moon said Monday that it would be “reprehensible” for Syria to consider using weapons of mass destruction such as chemical weapons. He said he hopes the international community monitors the situation to ensure such an attack does not happen.