The family of Nicole Mansfield, a convert to Islam from the US state of Michigan, say the FBI has confirmed her death. The British man is said to have been born in 1990.
The reports are unconfirmed but the westerners were said to have been with rebel forces when they were attacked by the Syrian army.
The same rights group, the UK-based Syrian Observatory for Human Rights, says the government controls most of Qusair following a fierce 12-day battle.
Syrian rebels have pleaded for military and medical aid in the town.
The Lebanese militant group Hezbollah has said it has joined the battle on the government's side from across the border.
In the central province of Homs where Qusair is located, as well as in Damascus, rebels have suffered setbacks around their supply routes.
Unverified amateur footage said to be from the district of Maskana in Aleppo purports to show people using their bare hands to recover bodies following heavy shelling by government forces.
Source: Euronews
