Syrian government forces are targeting civilians in barrel bomb attacks in Aleppo that have forced hospitals and schools to move underground, Amnesty International has said, describing the bombings as "crimes against humanity".

Barrel bombs -- containers packed with explosives and projectiles that are dropped from helicopters -- killed some 3,000 civilians in the northern Aleppo area last year, and have killed more than 11,000 in Syria since 2012, Amnesty said.

"I saw children without heads, body parts everywhere ... it was how I imagine hell to be," one factory worker told Amnesty in a report.

Another resident said the streets had been filled with blood and described Syria's second city as "the circle of hell".

Rights groups have long raised concerns about barrel bombs, and the UN Security Council adopted a resolution early last year condemning their use in populated areas, threatening "further steps" in the case of non-compliance.

Syrian President Bashar al-Assad said in February the Syrian air force did not use barrel bombs. US and European officials have said Mr Assad's denial is not credible.

Aleppo, near the Turkish border, is a major frontline in the Syrian war.

"Sheer terror and unbearable suffering has forced many civilians in Aleppo to eke out an existence underground to escape the relentless aerial bombardment of opposition-held areas by government forces," Amnesty said in the report.

Syria's four-year-old civil war began in 2011 when a government crackdown on a pro-democracy movement led to an armed uprising. Islamic State militants have taken advantage of the chaos to declare a caliphate in a swathe of Syria and Iraq.

The conflict has killed more than 220,000 people in Syria and uprooted some 7.6 million within the country. Nearly 4 million have fled to nearby countries.