WASHINGTON / DAMASCUS: U.S. Secretary of State John Kerry said Tuesday his government was “absolutely certain” the overwhelming majority of chemical attacks in Syria had been carried out by the regime, and was currently compiling evidence to back the claims.

Kerry said he was confident the government of Syrian President Bashar Assad was responsible for a “vast preponderance” of chemical attacks and that “everyone’s patience is wearing thin.”

He told reporters that he had spoken with Russian Foreign Minister Sergey Lavrov and said the United States was “engaged in a number of efforts, diplomatically and otherwise” on the matter.

During an unscheduled appearance during the State Department daily briefing, Kerry said the government was compiling evidence to support his claims. “I am absolutely certain, we are certain that the preponderance of those attacks have been carried out by the regime,” Kerry said, speaking via video from Boston. He added that some opposition groups might have had access to chemical materials and used them during the war, but stressed the alleged use of them by government forces.

“It has been significantly documented; it’s dropped from airplane. The opposition isn’t flying airplanes or helicopters,” he said.

In Damascus, Assad and U.N. peace envoy Staffan de Mistura agreed to continue talks to find a political solution to the ongoing conflict, Syria’s state news agency reported. At the end of their meeting, Assad and de Mistura “agreed ... to find a successful political solution in Syria and to return security and stability throughout Syria,” SANA said.

The agency said de Mistura briefed Assad “on the results of his consultations in Geneva with Syrians who represent various sides of Syrian society.”

On May 5, de Mistura launched a fresh round of consultative talks in Geneva with regional and local stakeholders in Syria’s conflict, including Iran, in a bid to kickstart political negotiations on the crisis.

Participants have included government officials and members of the opposition-in-exile National Coalition, as well as representatives from regional countries, policy experts and civil society groups.

The talks will continue throughout July, after which de Mistura will present an assessment to U.N. Secretary-General Ban Ki-Moon.

Assad and de Mistura also discussed the deaths of dozens of civilians on Monday in rebel rocket fire on government-controlled parts of Aleppo city, SANA said, referring to the deaths as a “massacre by terrorists.”

Monday’s toll was one of the highest for the regime-held part of the city since the Syria conflict began in 2011.

Assad warned that the world’s “silence about the crimes committed by terrorists will encourage them to continue their terrorism.”

“The whole world must wake up to the danger that this terrorism poses to security and stability, and it must take a clear and firm position against those who fund and arm and facilitate movement for terrorists,” SANA cited Assad as saying.

In a statement, de Mistura strongly condemned the deaths, but said they should not “justify in any case a retaliation on populated areas through barrel bombs by the Syrian government.” The use of barrel bombs has been criticized by human rights groups because of the indiscriminate death toll they cause among civilians when dropped on residential areas.

At least 16 people, among them 13 children, were killed in Syrian government airstrikes on a town in southern Deraa, according to the Syrian Observatory for Human Rights. Anti-regime activists and the Observatory said the strikes hit a center for study of the Quran in the town of Eastern Ghariyah.

One activist group in Deraa gave a figure of 21 as the death toll.

The Britain-based monitor accused the government of a “massacre,” saying a woman was also among those killed in the strikes.

It said the death toll was expected to rise further because a number of those wounded were in serious condition.

Eastern Ghariyah lies near the eastern edge of Deraa province, not far from the 52nd Brigade base that a rebel alliance seized from the regime a week ago.

The Southern Front alliance seized the base in a surprise attack, before pushing across the provincial border into Swaida and briefly overrunning the Thaaleh air base.

Regime forces expelled the rebels from Thaaleh Friday and a rebel leader Monday said the rebels had abandoned their campaign to seize the facility.

More than 230,000 people have been killed in Syria’s conflict, which began with anti-government demonstrations in March 2011 and descended into a civil war after a regime crackdown.

Rights groups have regularly criticized Syria’s government for indiscriminate fire on civilian areas. Rebel forces have also been accused of targeting civilian areas in parts of the country.