More than 42 civilians are reported to have been killed and dozens more wounded in air strikes launched by the Syrian government in al-Atareb, a city in Aleppo province in the country's northwest.

With violence continuing to escalate in the area, a government news agency reported that the Syrian army carried out operations against opposition fighters in locations across the country on Monday.

"There were around 27 air raids," one local resident told Al Jazeera on Monday, referring to a night of air strikes reportedly carried out by Syrian government and Russian forces.

Al Jazeera's Bernard Smith, reporting from Gaziantep in neighbouring Turkey, said al-Atareb "has been targeted many times during this country's five-year civil war ... but this attack was the biggest yet, according to people there.

"Four of the six medical facilities hit over the weekend are now closed."

Marwan Kabalan, a research associate at the Doha Institute's Arab Center for Research & Policy Studies, says Syrian government forces are trying to wear down opposition fighters in Aleppo.

Referring to Russian-US negotiations over cooperation in Syria, Kabalan told Al Jazeera the Syrian government is "trying to encircle and lay siege to Aleppo before any understanding can be reached between the Russians and the Americans".

Against this backdrop of continued fighting in Aleppo and other provinces, Stephen O'Brien, the UN humanitarian chief, called on Monday for immediate "unconditional and unhindered" humanitarian access to an estimated 5.5 million Syrians in besieged or difficult-to-reach areas.

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"It is well within the power of all parties and those who back them to minimise civilian casualties and avoid further crimes and atrocities," he told the UN.

"They must do so. Civilians and civilian infrastructure are not pawns to be sacrificed but are especially protected under international law."

The official state-run SANA news agency reported on Monday that a woman and her child were killed on Monday in Aleppo when fighters fired shells on a government-held neighbourhood.

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A car bomb blast hit Damascus on Monday, Syrian state news agency SANA reported, without giving details of casualties.

An AFP journalist heard the powerful explosion, which SANA said hit the modern, upmarket Kafar Sousse area in the southwest of the wartorn country's capital.

The official Al-Ikhbariya television channel said at least one person was injured, while the Britain-based Syrian Observatory for Human Rights, which also reported the car bomb attack, spoke of casualties without giving details.

SANA also reported that the Syrian army on Monday carried out operations against opposition fighters in areas across the country, including the provinces of Aleppo, Deraa, Deir Ez-Zor, Hama and Homs.

The Syrian uprising started with largely unarmed protests against President Bashar al-Assad in March 2011, but it quickly turned into a full-blown civil war that has since continued unabated.

The UK-based Syrian Observatory for Human Rights network estimates that more than 280,000 people have died in the five years of bloodshed.