United Nations Secretary-General Ban Ki-moon has warned of an unprecedented "humanitarian catastrophe" in Syria's Aleppo and urged Russia and the US quickly to reach a deal on a ceasefire in the city and elsewhere in the country.

Fighting for control of Aleppo, split between its government-held west and rebel-held eastern neighborhoods, has intensified in recent weeks causing hundreds of deaths and depriving many civilians of power, water and vital supplies.

"In Aleppo we risk seeing a humanitarian catastrophe unprecedented in the over five years of bloodshed and suffering in the Syrian conflict," Ban told the UN Security Council in his latest monthly report on aid access, seen by Reuters.

Aleppo is one of the bastions of the rebellion to oust Syrian President Bashar al-Assad, whose army is backed on the ground by Shi'ite Muslim militias from neighboring countries and from the skies by Russian air strikes.

"The fight for territory and resources is being undertaken through indiscriminate attacks on residential areas, including through the use of barrel bombs, killing hundreds of civilians, including dozens of children," Ban said in the U.N. report.

"All parties to the conflict are failing to uphold their obligation to protect civilians," he said.

Ban reiterated a UN call for at least a 48-hour humanitarian pause in fighting in Aleppo for aid deliveries and also pushed Moscow and Washington to rapidly reach a deal on a ceasefire.

Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry on Tuesday discussed securing a ceasefire, the Russian Foreign Ministry said.

Russia used Iran as a base from which to launch air strikes against Syrian militants for the first time on Tuesday. The Russian Defence Ministry said it takes great care to avoid civilian casualties in its air strikes.

The Syrian Observatory for Human Rights, a U.K.-based war monitor, said heavy air strikes on Tuesday had hit many targets in and around Aleppo and elsewhere in Syria, killing dozens.

The United States had been targeting Islamic State militants in Syria with air strikes for nearly two years.