The Syrian military has stepped up ballistic missile strikes on rebel-held areas, including four attacks in the north last week that killed more than 141 people, including 71 children, Human Rights Watch said on Tuesday.
"I have visited many attack sites in Syria, but have never seen such destruction," said Ole Solvang, emergencies researcher at the New York-based group, who has inspected the four areas.
"Just when you think things can't get any worse, the Syrian government finds ways to escalate its killing tactics."
Syrian Information Minister Omran al-Zoabi denied on Sunday that the government was using Scud missiles in the conflict, the Russia Today Arabic website reported.
As the now-militarised uprising against President Bashar al-Assad nears its two-year mark, the armed forces have used tanks, artillery and air strikes on residential areas as they battle insurgents for control of Syria's main cities.
The United Nations estimates 70,000 people have been killed in a conflict that has raised tensions across the Middle East.
A Human Rights Watch statement said there was no sign of fighters or rebel bases in areas hit by ballistic missiles, only civilians, meaning that the attacks were unlawful.
It said each strike had completely destroyed 15 to 20 houses and that the total death toll was probably higher than the 141 people it had been able to document.
"Using ballistic missiles against its own people is a new low, even for this government," Solvang said.
Three of the places hit were in an opposition-controlled eastern part of Aleppo, where rebel forces took half the city in July and August but are now stuck in a stalemate with the army. The fourth missile struck Tel Rifat, a town north of Aleppo.
The statement quoted a resident of the Ard al-Hamra neighbourhood in Aleppo, which was struck on Feb. 22.
"I was having evening tea... with my brother as I used to every evening in his house. Just after I had left at 9:15 p.m. the sky was lit up by a tremendous flash and all air was sucked away. The explosion was deafening," the resident said.
"When I ran back, my brother's house was gone. We managed to find my five young nieces and nephews, aged between 3 and 17 years old. They were all dead under the rubble. We still have not found my brother," he said.
Human Rights Watch says the extent of damage from a single strike, the absence of aircraft from the area at the time and activists' reports of missiles being launched from a military base near Damascus overwhelmingly suggest that government forces had struck these areas with ballistic missiles.
However, it did not find missile remnants at the attack sites, and so was unable to identify the exact weapons used.
NATO said in December that Assad's forces had fired Scud-type missiles on several occasions. It did not specify where they landed but said their deployment was an act of desperation.
Opposition activists have counted more than 30 ballistic missile attacks since December. Several of the missiles landed harmlessly in fields.