One of the main hospitals in Syria's northern city of Aleppo has been forced to close indefinitely after being targeted by rockets and barrel bombs dropped by government helicopters, an international humanitarian group has said.

Doctors Without Borders (MSF) said on Monday that the private al-Sakhour hospital, serving around 400,000 people as one of the only hospitals in east Aleppo, halted all activities after being bombed twice on consecutive days last week.

"It is unclear when or if the hospital will be operative again" as it was severely damaged, an MSF statement said.

The hospital's staff are Syrian, but it receives medical equipment from MSF every three months, an MSF representative told the AFP news agency.

"The next delivery was supposed to be in June, but we don't know if it will happen," he said.

Raquel Ayora, MSF's director of operations, said: "We renew our appeal to the warring parties to respect civilians, health facilities and medical staff."

According to the Syrian Observatory for Human Rights (SOHR) , barrel bombs - highly destructive and indiscriminate crude weapons made of old containers packed with explosives - struck near the hospital on April 28 and 30.

The UK-based monitoring group has documented a total of more than 11,000 air raids carried out by the regime warplanes across the country since the beginning of this year.

Atrocities in Aleppo

Also, rights body Amnesty International on Tuesday accused the government forces of crimes against humanity in Aleppo. It however, said that rebels have also committed abuses in the contested northern city.

Aleppo, once Syria's largest city and commercial capital, has been a major battleground in the country's civil war since rebels launched an assault there in mid-2012 that ultimately left the city carved into government- and opposition-held halves.

Widespread atrocities, in particular the vicious and unrelenting aerial bombardment of civilian neighborhoods by government forces, have made life for civilians in Aleppo increasingly unbearable Philip Luther, Amnesty

In a new report, Amnesty International sharply condemned the government's reliance on barrel bombs against rebel-held neighbourhoods in the city.

"Widespread atrocities, in particular the vicious and unrelenting aerial bombardment of civilian neighborhoods by government forces, have made life for civilians in Aleppo increasingly unbearable," said Amnesty's Philip Luther.

"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."

Amnesty said barrel bomb attacks have killed more than 3,000 civilians in Aleppo governorate last year, and more than 11,000 in the country since 2012.

Meanwhile, in the hope of reviving stalled dialogue to find a possible solution to the conflict, the UN's peace envoy for Syria, Staffan de Mistura, announced on Tuesday the start of wide-ranging consultations in Geneva with regional and domestic players.

De Mistura said that talks with the Syrian government and some 40 groups - including "political, military actors, women, civil society, victims, the diaspora" - would also rope in some 20 regional and international players.

Iran, which backs the government of President Bashar al-Assad, will also be involved in the talks.

The consultations would be held on a one-to-one basis between the UN and the separate players.