The rocket fire followed the government's capture on Monday of the Khaldiyeh district of the flashpoint city that had been in the hands of mainly Sunni rebels for the past two years.
The ammunitions depot blew up in the pro-regime Wadi al-Zahab district "after rocket fire, most probably by rebels," said the London-based Observatory, which relies on a network of activists and medics for its information.
It said the depot was operated by pro-regime militias.
Rockets also crashed into Akruma and Al-Nozha, all districts in southern Homs inhabited mainly by members of President's Bashar al-Assad's Alawite community.
A doctor living in Akruma told AFP that residential districts were also hit, causing the collapse of apartment blocks.