U.S. 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, Reuters wrote.
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 on Friday that Moscow was ready to consider more ways to normalize the situation in Aleppo.
But in a phone call with U.S. Secretary of State John Kerry, Lavrov criticized 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.
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 favor of Assad, who is also supported by Iranian ground forces and Shi'ite militia from Lebanon and Iraq.
The Kremlin said on Friday 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.
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."
“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."
Read alsoU.S. weighs tougher response to Russia over Syria crisis: officials – ReutersThe 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 armored 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 defense ministry did not immediately respond to a request from Reuters for comment.
