Syrian government forces are targeting civilians in barrel bomb attacks in Aleppo that have forced hospitals and schools to move underground, rights group Amnesty International says in a new report that describes the bombings as "crimes against humanity".

Tuesday's report, "Death everywhere: War crimes and human rights abuses in Aleppo" details war crimes and other abuses being committed in the city by government forces and armed opposition groups on a daily basis, and concludes that some of the government's actions in Aleppo amount to crimes against humanity.

It says many civilians in Aleppo have been forced to eke out an existence underground to escape a relentless aerial bombardment of opposition-held areas by government forces.

"Widespread atrocities, in particular the vicious and unrelenting aerial bombardment of civilian neighbourhoods by government forces, have made life for civilians in Aleppo increasingly unbearable," said Philip Luther, director of Amnesty International's Middle East and North Africa Programme.

"These reprehensible and continual strikes on residential areas point to a policy of deliberately and systematically targeting civilians in attacks that constitute war crimes and crimes against humanity."

Attacks using barrel bombs - oil barrels, fuel tanks or gas cylinders packed with explosives, fuel, and metal fragments dropped from helicopters - killed more than 3,000 civilians in Aleppo last year, and more than 11,000 in Syria since 2012, Amnesty said.

A local surgeon said the level of injuries he had seen caused by barrel bombs was unprecedented: "Barrel bombs are the most horrible and hurtful weapon… [We deal with] multi-trauma, so many amputations, intestines out of the body, it's too horrible," he said.

Rights groups have also accused the Syrian government of filling barrel bombs with chlorine gas, a banned chemical weapon, to bomb rebel-held areas of the country.

Meanwhile, the UN Children's Fund (UNICEF) announced on Tuesday that it had managed to deliver aid to the devastated Damascus suburb of Yarmouk, a former Palestinian refugee camp, for the first time in two years.

Fighters from the Islamic State of Iraq and the Levant (ISIL) seized large parts of Yarmouk last month and have been battling other armed groups inside.

UNICEF had been unable to reach these areas previously because it lacked permission from the Syrian government to cross over.

Yarmouk was already a civil war battleground before the ISIL attack, having been reduced to rubble from street fighting, air attacks and shelling.

Also on Tuesday, the UN envoy to Syria said he had modest hopes of an outcome for next week's Syrian peace talks in Geneva, describing the talks as a "reality check" and saying that they would be take the form of low-key, "one-on-one" meetings.

Special Envoy of the United Nations' Secretary-General for Syria, Staffan de Mistura, told a news conference in Geneva that the talks were not a new round of negotiations but a series of individual meetings with "as many actors involved in the Syrian conflict as possible" to get a sense of whether another round of negotiations was feasible.

"The idea is still on the table: a freeze. Stopping on both sides, on all sides the use at least of the heavy weapons," de Mistura said.

"But the situation unfortunately is not conducive to that. In order to have a freeze, you have to have all sides, at least at the beginning, agreeing on giving it a chance."

Iran attending

Iran is to take part in the talks for the first time. The country's participation is important because it is a key ally of the Syrian government and did not take part in the two earlier rounds of negotiations on Syria that failed.

Iran has been careful to say it would be interested in attending without preconditions, namely, the insistence by some parties that President Bashar Assad must go.

Groups fighting in Syria, including ISIL and the al-Nusra Front, have not been invited.

Al Jazeera's Paul Brennan, who attended the Geneva press conference, said at least 40 separate groups, including the Syrian government, and 20 international stakeholders would attend the negotiations.

Brennan said de Mistura was modest in setting his aims.

"The indications are not positive," Brennan said.