It detonated as a vehicle with the seven on board passed through a desert road outside the historic city of Palmyra, the Britain-based Syrian Observatory for Human Rights said.

"It destroyed the vehicle and killed all the passengers inside, which included two women, a child, and four men," said the monitor, which relies on a network of sources inside Syria.

Explosives left in fields, along roads or even in buildings by all sides in Syria's decade-long conflict have wounded thousands of civilians and killed hundreds of others.

The Landmine and Cluster Munition Monitor recorded 182 casualties last year in Syria, mainly from cluster munition remnants.

The figure accounted for nearly half of the 360 cluster munition casualties documented across the world in 2020, according to the watchdog.

It has recorded a total of 4,099 cluster munition casualties in Syria, including 2,102 in attacks and 1,997 from cluster munition remnants.

Across Syria, one in three populated communities are thought to be contaminated by explosive ordnance, the United Nations said in March.

One in two people are at risk from explosive ordnance contamination, it added in a report.

Syria's war has killed nearly half a million people and displaced millions since it began with the brutal repression of anti-government protests in 2011.