Syria's regime intensified air strikes on rebel-held areas of Aleppo province Monday as the UN backed a call by its top aid official for a humanitarian truce in the city.

The call for aid to reach desperate civilians trapped by a regime siege in Aleppo came on the eve of a meeting between a UN envoy, US and Russian officials to try to revive peace talks.

UN aid chief Stephen O'Brien branded the siege on rebel-held parts of Aleppo as "mediaeval and shameful", and called for 48-hour weekly humanitarian truces to prevent it from taking hold.

O'Brien warned that food supplies in eastern Aleppo -- home to at least 200,000 people -- were expected to run out by the middle of August.

"The international community simply cannot let eastern Aleppo city become yet another -- and by far the largest -- besieged area," O'Brien said.

Nearly 600,000 people are estimated to live under siege in Syria, most of them encircled by forces of President Bashar al-Assad's regime, whose approval the United Nations says is needed to deliver aid by air.

Britain, France and the United States quickly endorsed O'Brien's call. Japan's Ambassador Koro Bessho, who holds the council presidency, said there was "overwhelming support for the idea" among the 15 council members.

French ambassador to the UN Francois Delattre compared Aleppo's plight to that of Sarajevo during the Bosnian war, and said "the Security Council simply cannot accept such war crimes -- yes war crimes -- to repeat again".

Britain's UN Ambassador Matthew Rycroft said: "Enough is enough now."

- Talks about talks -

On Monday, air strikes and barrel bombs killed 22 civilians in rebel-held parts of the northern province, and rebel rocket fire on government areas killed three more, a monitor said.

The bombardment came as UN Syria envoy Staffan de Mistura was expected to meet top US and Russian officials in Geneva on Tuesday with the aim of reviving peace talks to end the five-year conflict.

More than 280,000 people have been killed in Syria and more than half its population has been displaced since the conflict began with the brutal repression of anti-government protests in 2011.

US mission spokesman Paul Patin said Washington's special envoy for Syria Michael Ratney would be at Tuesday's meeting in Geneva.

Russia's Ria-Novosti news agency said deputy foreign minister Gennady Gatilov would represent Moscow, a key ally of the Syrian regime.

US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov were also due to meet Tuesday on the sidelines of an Asian summit in Vientiane, Laos.

The two announced earlier in July an agreement on "concrete steps" to salvage a failing nationwide ceasefire in Syria, a key step before negotiations can resume.

The February ceasefire between the regime and non-jihadist rebels -- which was brokered by the US and Russia -- remains largely in tatters.

The Britain-based Syrian Observatory for Human Rights said 10 civilians were killed in air strikes on the rebel-held town of Atareb in Aleppo province on Monday.

- Barrel bomb attacks -

It said the strikes, believed to have been carried out by Russian warplanes, hit several locations including a market area.

Footage obtained by AFP showed a civil defence unit trying to put out a fire in the rubble of one collapsed building after the strikes as people combed through debris.

One group was seen trying to extract medicine from what appeared to be the remains of a pharmacy or clinic.

In Aleppo city, at least 12 civilians were killed in barrel bomb attacks by government forces on rebel-held neighbourhoods, the Observatory said.

In the Al-Mashhad district, civil defence workers struggled to retrieve trapped survivors, an AFP journalist said.

One boy was pulled out alive but the rest of his family were killed and their bodies remained under a collapsed building, he added.

Once Syria's economic powerhouse, Aleppo has been roughly divided between government control in the west and rebel control in the east since mid-2012.

This month regime forces seized parts of the only remaining supply route into the city's east, cutting off opposition districts from the outside world and prompting food shortages and spiralling prices.

The Observatory said at least three civilians were killed by rebel fire on western Aleppo on Monday.

Syrian state news agency SANA said a pregnant woman and her child were killed.

SANA also reported that four people had been wounded on Monday in rebel rocket fire on central Damascus.

Rebel rockets also hit Old Damascus late Sunday after air raids struck four makeshift hospitals in Aleppo's rebel districts.

burs-ah/hkb/srm