The United Nations, not Syria, must investigate the Houla massacre, Human Rights Watch said adding that witness testimony suggest government forces were responsible for the carnage.
"Kofi Annan should push Syria's government to allow the UN-appointed Commission of Inquiry access into the country to investigate," the massacre in which at least 108 people were killed, including 49 children.
"As long as gunmen can operate with impunity, the horrors in Syria will continue," warned Sarah Leah Whitson, Middle East and North Africa director for HRW.
Syria has denied any responsibility and said it will launch an investigation into the carnage that took place in Houla, central Syria, on Friday and Saturday.
"There’s no way a Syrian military commission can credibly investigate this horrendous crime," said Whitson.
HRW said it interviewed survivors and local activists who said "the Syrian army shelled the area on May 25, and armed men, dressed in military clothes, attacked homes on the outskirts of town and executed entire families."
"All of the witnesses stated the armed men were pro-government, but they did not know whether they were members of the Syrian army or a pro-government militia, locally referred to as shabiha," the watchdog said.
According to HRW, "most of those killed belonged to the Abdel Razzak family."
"Local activists provided Human Rights Watch with a list of 62 dead members from the Abdel Razzak family," it said.
An elderly survivor from the Abdel Razzak clan told HRW she heard gunshots at 6.30 pm (1530 GMT) on Friday, and then a man who was "shouting and yelling at my family."
"After three minutes, I heard all my family members screaming and yelling. The children, all aged between 10 and 14, were crying," she is quoted as saying.
After the soldiers left, the woman she looked into one of the rooms of the house and "saw all of my family members shot... in their bodies and their head."
A 10-year-old boy from the same family told HRW he saw "men in military clothes shoot his 13-year-old friend" at close range, and aim at his mother and sister.