Western leaders have condemned Syria's Bashar Assad and Russia's Vladimir Putin for obstructing the supply of humanitarian aid to Aleppo.
Prime Minister Theresa May joined the leaders of the US, Canada, France, Germany and Italy to issue a joint statement calling for an immediate ceasefire to allow aid to reach eastern Aleppo, where Syrian regime forces have been bombarding rebel-held areas.
The statement accused Russia and Iran of failing to support a political process to end the bloodshed in Syria, instead concentrating on supporting the Assad regime.
The leaders said: "A humanitarian disaster is taking place before our very eyes. Some 200,000 civilians, including many children, in eastern Aleppo are cut off from food and medicine supplies.
"Aleppo is being subjected to daily bombings and artillery attacks by the Syrian regime, supported by Russia and Iran.
"Hospitals and schools have not been spared. Rather, they appear to be the targets of attack in an attempt to wear people down. The images of dying children are heart- breaking.
"We condemn the actions of the Syrian regime and its foreign backers, especially Russia, for their obstruction of humanitarian aid, and strongly condemn the Syrian regime's attacks that have devastated civilians and medical facilities and use of barrel bombs and chemical weapons."
The leaders highlighted UN Secretary-General Ban Ki-moon's statement that war crimes had been committed and said "there must be no impunity for those responsible", suggesting that further sanctions could be imposed against allies of the Assad regime.
"We call on the UN to investigate respective reports and gather evidence to hold the perpetrators of war crimes to account. We are ready to consider additional restrictive measures against individuals and entities that act for or on behalf of the Syrian regime," the joint statement said.
Russia has repeatedly blocked action in the UN Security Council over Syria and the leaders said Moscow's actions had left the organisation "unable to do its work and put an end to the atrocities".
The joint statement pointed the finger at both Moscow and Tehran for frustrating efforts to find a peaceful end to the brutal civil war.
"The regime's refusal to engage in a serious political process also highlights the unwillingness of both Russia and Iran to work for a political solution despite their assurances to the contrary," the leaders said.