The London group posted video on its website showing the bodies of young men with their hands tied, the Financial Times reported. The men were in their 20s and had been shot.
Rebel groups said the killing was done by forces loyal to President Bashar Assad.
The observatory said there were 65 victims. Another group, the Local Co-Ordination Committees, said there were 80.
The state-run Syrian Arab News Agency said Islamist fighters of the Nusra Front were to blame for the latest killings, which it said were part of "a series of brutal massacres perpetrated by the terrorist groups against unarmed civilians."
The Syrian Network for Human Rights, also based in London, said it had documented 150 deaths across Syria Tuesday, including 14 children and four women. The group, whose casualty numbers could not be verified, said 69 of the deaths were in Aleppo.
Syrian dissidents said the victims appeared to be Sunni Muslims, The New York Times reported.
The U.N. High Commissioner for Refugees said Tuesday the flow of refugees from Syria into neighboring countries has increased, the Times said. Sybella Wilkes, a UNHCR spokeswoman, said there are now more than 700,000, up from 500,000 two months ago.
Turkey is hosting the largest number of refugees, followed by Jordan and Lebanon, but there are also some in Egypt and other northern African countries.
"We are trying to clear a backlog of people because the numbers have gone up so dramatically," Wilkes said.
The agency reported Friday 30,000 refugees made their way to Jordan in January, up from about 16,000 in December.
Rebels Tuesday announced military advances in eastern Syria, the Times said. They said they had made a strike at a security post in Deir al-Zour.
President Obama announced an additional $155 million in U.S. humanitarian aid for refugees and for those who remain in Syria.
"Here, I want to speak directly to the people of Syria," Obama said. "This new aid will mean more warm clothing for children and medicine for the elderly; flour and wheat for your families and blankets, boots and stoves for those huddled in damaged buildings.
"It will mean healthcare for victims of sexual violence and field hospitals for the wounded. Even as we work to end the violence against you, this aid will help address some of the immediate needs you face each day."