Russian forces are due to suspend military action in the northern Syrian city of Aleppo shortly to allow in desperately needed humanitarian aid.

It is not clear if Syrian rebel groups will also cease fire during the daily three-hour window.

But UN officials say that is too short to take in enough to help the large numbers of people in need.

Intense fighting has been continuing in Aleppo between rebels and Russian-backed Syrian government forces.

There are also reports of a toxic gas attack on a rebel-held area. Medical staff say four people died and many other were injured. The gas is thought to have been chlorine dropped in a barrel bomb.

Earlier the last doctors in the rebel-held east of the city appealed to US President Barack Obama to come to the aid of the 250,000 civilians there.

The remaining Aleppo doctors say in their letter that in the five years since the uprising against President Bashar al-Assad began, they have "borne witness as countless patients, friends and colleagues suffered violent, tormented deaths".

"The world has stood by and remarked how 'complicated' Syria is, while doing little to protect us. Recent offers of evacuation from the regime and Russia have sounded like thinly veiled threats to residents - flee now or face what fate?"

They say that in the past month there have been 42 attacks on medical facilities in Syria, 15 of them on hospitals where they work.

On Monday, the UN said countless civilians had been killed or injured over the past few weeks in the city, and that the targeting of hospitals and clinics had continued unabated.

Moreover, attacks on civilian infrastructure had left more than two million people without electricity or access to the public water network for several days, it added.

(BBC)