Syrian opposition activists say security forces and rebels engaged in fierce battles in Damascus and Aleppo on Tuesday, as President Bashar al-Assad replaced key officials killed in a bombing last week.

Opposition groups say government forces staged a series of raids in Damascus neighborhoods. They also report heavy shelling and gunfire in Aleppo, where 15 people were killed as security forces put down a prison mutiny.

The opposition says government shelling is underway in several other regions, including the southern town of Herak where at least six children were killed in blasts.

Syrian government officials say President Assad has appointed several officials to top security posts, including Ali Mamlouk as national security chief. He replaces General Hisham Ikhtiya, one of four high-level officials who died as a result of a rebel blast in Damascus on Wednesday.

In another development, U.S. 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.