Human Rights Watch said Tuesday it has identified nearly 1,500 sites where the Syrian government has carried out indiscriminate air attacks over the past year using barrel bombs and other improvised weapons that have killed or injured thousands of people.

The human rights group accused President Bashar Assad of lying when he said his government was not using barrel bombs and said in a new report that the attacks have had “a devastating impact on civilians.”

Human Rights Watch said that by examining satellite imagery it identified some 450 major damage sites in 10 towns and villages held by rebel groups in the Daraa governorate in the south and over 1,000 in war-ravaged Aleppo, Syria’s largest city. It also examined video and photographic evidence and interviewed witnesses to document the attacks.

Nadim Houry, the group’s deputy Middle East and North Africa director, told a news conference that the majority of deaths are still caused by the Assad government, and the bombing is the “key cause of the displacement crisis in Syria.”

The four-year-old Syrian conflict has killed 220,000 people, according to U.N. estimates, and displaced 6.5 million inside the country and more than 3 million people to neighboring countries.

In an interview with the BBC on Feb. 10, Assad denied his forces have used barrel bombs, insisting that the army uses bullets, missiles and bombs. “There are no barrel bombs; we don’t have barrels,” he insisted.

Philippe Bolopion, United Nations director at Human Rights Watch, told reporters “we know he is simply lying.”

The government’s use of barrel bombs, usually dropped by helicopters, has been widely documented by international human rights organizations and residents of opposition-held areas in Syria.

The Syrian Network for Human Rights reported on Feb. 22 that 6,163 civilians – including 1,892 children and 1,720 women – have been killed by the crude explosive devices since the Security Council adopted a resolution a Feb. 22, 2014 which demanded that all parties to the conflict in Syria end the indiscriminate use of barrel bombs and other weapons in populated areas.

The Violations Documentation Center, a local monitoring group, has documented 609 civilian deaths in Daraa and 2,575 civilian deaths in Aleppo governorate over the past year from aerial attacks, Human Rights Watch said.

The rights group urged the Security Council to impose an arms embargo on Syria’s government and any group implicated in widespread human rights abuses.

“We believe an arms embargo would deal adequately with the issue of barrel bombs,” Bolopion said, because it would target shipments of spare parts, fuel for aircraft and other key items. SAPA