The airstrikes came after months of fighting on the outskirts of the city in Homs province near the Lebanese border, said Abu Ali, an opposition spokesman.
"The Syrian Army has tightened its siege on terrorists and is attacking their headquarters in the southern part of Qusayr," state media reported.
The BBC reported opposition forces said 16 people died Sunday in the conflict, although the Syrian Observatory and Local Coordination Committees activist groups confirmed the deaths of 13 people and 300 injuries.
Recapturing Qusayr would ensure access from the capital to the coast for government forces, the BBC said. For the rebels, holding the city means access to neighboring Lebanon.
Meanwhile, the opposition has accused Hezbollah fighters from Lebanon of joining Syrian government troops in Sunday's assault, CNN reported. Syrian President Bashar Assad has denied working with Hezbollah troops in the past.