Russia is sending more warplanes to Syria to ramp up its campaign of air strikes, a Russian newspaper reported today, as Moscow defied global censure over an escalation that Western countries say has torpedoed diplomacy.

US President Barack Obama and German Chancellor Angela Merkel described the Russian and Syrian bombing of the city of Aleppo as "barbarous", the White House said after the two leaders spoke by telephone.

Fighting intensified a week into a new Russian-backed government offensive to capture all of Syria's largest city and crush the last remaining urban stronghold of the rebellion.

Moscow and its ally, Syrian President Bashar al-Assad, spurned a ceasefire agreed this month to launch the offensive, potentially the biggest and most decisive battle in the Syrian civil war which is now in its sixth year.

Russian Foreign Minister Sergei Lavrov said today that Moscow was ready to consider more ways to normalise the situation in Aleppo.

But in a phone call with US Secretary of State John Kerry, Lavrov criticised Washington's failure to separate moderate rebel groups from those the Russians call terrorists, which had allowed forces led by the group formerly known as the Nusra front to violate the truce brokered by Moscow and Washington.

Civil Defense members search for survivors at a site hit by an airstrike in Aleppo. Photo: Reuters/Abdalrhman Ismail

The call came a day after Kerry said there was no point pursuing further negotiations with Russia over Syria "in the context of the kind of bombing taking place".

Western countries accuse Russia of war crimes, saying it has deliberately targeted civilians, hospitals and aid deliveries in recent days to crush the will of 250,000 people trapped inside Aleppo's besieged rebel-held sector.

Moscow and Damascus say they have targeted only militants.

Hundreds of people have been killed in the bombing and many hundreds more wounded, with little access to treatment in hospitals that lack basic supplies.

Residents say the air strikes are unprecedented in their ferocity, deploying heavier bombs that flatten buildings on top of the people huddled inside.

Russia joined the war exactly a year ago, tipping the balance of power in favour of Assad, who is also supported by Iranian ground forces and Shi'ite militia from Lebanon and Iraq.

The Kremlin said there was no time frame for Russia's military operation in Syria. The main result of Russian air strikes over the past year is "neither Islamic State, nor al Qaeda nor the Nusra Front are now sitting in Damascus", Kremlin spokesman Dmitry Peskov told reporters.

"REALITY A NIGHTMARE"

Britain's Special Representative to Syria, Gareth Bayley, said: "From Russia's first air strikes in Syria, it has hit civilian areas and increasingly used indiscriminate weapons, including cluster and incendiary munitions."

US President Barack Obama, pictured here during the funeral of former Israeli leader Shimon Peres, called Russia's airstrikes on Aleppo 'barbarous'. Photo: Reuters/Abir Sultan

"Today, the reality in Syria is a nightmare. Aleppo is besieged again, with vital necessities such as water, fuel, and medicine running out for hundreds of thousands. Civilian infrastructure, including schools and hospitals, are being attacked."

The Izvestia newspaper reported that a group of Su-24 and Su-34 warplanes had arrived at Syria's Hmeymim base.

"If need be, the air force group will be (further) built up within two to three days," it quoted a military official as saying. "Su-25 ground attack fighters designated to be sent to Hmeymim have already been selected in their units and their crews are on stand-by, awaiting orders from their commanders."

The Su-25 is an armoured twin-engine jet which was battle-tested in the 1980s during the Soviet war in Afghanistan. It can be used to strafe targets on the ground, or as a bomber. Russia's defence ministry did not immediately respond to a request from Reuters for comment.

BATTLEFIELD VICTORY

After months of intensive diplomacy with Russia, conducted despite the scepticism of other senior Obama administration officials, Kerry reached agreement three weeks ago on a ceasefire. But it collapsed within a week, and Moscow and Damascus swiftly launched the latest escalation.

Western officials believe Moscow's decision to spurn the truce signals the Kremlin believes Assad's government can now win a decisive victory on the battlefield, after years of mostly stalemated war that killed hundreds of thousands of people and made half of Syrians homeless.

Syrian government forces and rebels fought battles today in the city centre and north of Aleppo, where government troops had re-captured a camp for Palestinian refugees yesterday that had already changed hands once since the start of the attack.

The sides gave conflicting accounts of the outcome of today's fighting. North of the city, the military said it had captured territory around the Kindi hospital near the refugee camp. Rebel sources denied the army had advanced there.

In the city centre, the military said it had advanced in the Suleiman al-Halabi district. Rebel officials said troops had moved forward but had subsequently been forced to withdraw.

People walk on the rubble of damaged buildings in the rebel held area of al-Kalaseh neighbourhood of Aleppo, Syria. Reuters/Abdalrhman Ismail

A Syrian military source said government forces captured several buildings in the area and were "continuing to chase the remnants of the terrorists fleeing them". One of the rebel officials said government forces had "advanced and then retreated", losing "a number of dead".

The multi-sided Syrian civil war pits Assad, a member of the Alawite sect supported by Iran and Shi'ite militia, against rebels mainly drawn from Syria's Sunni Muslim majority, backed by Saudi Arabia and other regional Sunni states.

The rebellion includes several groups inspired by or linked to al Qaeda, and helped give rise to Islamic State, a hardline Sunni group that broke away from al Qaeda and declared a caliphate in swathes of Syria and Iraq.

Russia, which has been allied to Assad and his father since the Cold War, says the only way to defeat Islamic State is to support Assad. Washington and its European allies say the Syrian leader has too much blood on his hands and must leave power so the rest of the country can unite against the militants.

United Nations Secretary-General Ban Ki-moon announced that he would establish an internal UN board of inquiry to investigate a deadly attack on a humanitarian aid convoy in Syria last week and urged all parties to fully cooperate.