Human Rights Watch Tuesday accused Syrian government forces of firing at least four ballistic missiles at rebel-held areas in the northern province of Aleppo in February, dpa reported.
The missile strikes killed more than 141 people, among them 71 children, the US-based rights group said.
"Using ballistic missiles against its own people is a new low, even for this government," said Ole Solvang, emergencies researcher at Human Rights Watch, who visited the sites.
Solvang said that he visited the four sites. All were residential neighbourhoods with no sign of any military targets in their vicinity.
"I have visited many attack sites in Syria, but have never seen such destruction," said Solvang.
Rebels have accused the regime of using Russian-made Scud missiles to shell residential areas and called upon the world community to confront Syria about such attacks. The government has denied using the missiles.
The HRW report came as Russian Foreign Minister Sergei Lavrov said that extremist stances among the opposition were blocking its participation in talks with the government.
"Several days ago, it seemed to us that the parties were ready for a dialogue without preconditions, but then these statements were refuted. Extremists have prevailed at this stage," Lavrov said in comments quoted by Russia's Itar-Tass news agency.
Lavrov added that there was "a growing understanding of a need to influence both the government and the opposition" among international players.
"We will speak about this with US Secretary of State John Kerry in Berlin today," he said.
US diplomatic efforts yesterday succeeded in convincing the Syrian National Coalition to attend the international "Friends of Syria" conference, due to take place on Thursday in Rome.
The group had previously announced it would boycott the talks because of "the world's silence" about the ongoing violence in Syria.