Defectors from Syria's security forces have described receiving orders from their superiors to fire live rounds at protesters to disperse them, according to Human Rights Watch.

The New York-based rights body released a statement on Saturday detailing interviews with eight soldiers and four members of secret security agencies it said had defected to the opposition since anti-government protests erupted in March.

Omar al-Issawi, director of advocacy and communications for the Middle East and North Africa division of Human Rights Watch, told Al Jazeera in an interview on Sunday that he is certain all the detailed information provided by the defectors is "absolutely" accurate and provides "overwhelming evidence" of massacres of civilians by security forces across Syria.