Ongoing attacks by Russia and the Syrian regime on the northern city of Aleppo are "acts that beg for an appropriate investigation of war crimes," US Secretary of State John Kerry said Friday in Washington.
"Russia and the [Syrian] regime owe the world more than an explanation about why they keep hitting hospitals and medical facilities and children and women," Kerry said ahead of bilateral discussions with French Foreign Minister Jean-Marc Ayrault.
Those who commit such attacks should be held accountable, Kerry said.
"They're beyond the accidental now, way beyond," the US diplomat said. "This is a targeted strategy to terrorize civilians and to kill anybody and everybody who is in the way of their military objectives."
Russian Deputy Foreign Minister Sergei Ryabkov rejected Kerry's call for a war crimes investigation as "intolerable" late Friday evening in Moscow.
Earlier this week, the US suspended its cooperation with Russia on Syria, citing the failure of a ceasefire and Moscow's inability to end the violence in the war-torn country.
Russia, a major ally of Syrian President Bashar al-Assad, has been bombing militant groups in Syria for more than a year.
German Chancellor Angela Merkel on Friday called for an end to the heavy fighting in Aleppo.
"It's terrible what is happening there. We have to do all in our power to reach a ceasefire, and in particular to really take care of the people in need," Merkel told a gathering of party supporters in the city of Magdeburg.
Forces backing Syrian President Bashar al-Assad, supported by Russian air power, are engaged in an all-out battle to seize control of the rebel-held eastern part of the city.
"I can only appeal to Russia. Russia has a lot of influence over al-Assad. We have to put an end to this ghastly crime as quickly as possible," Merkel said.
Meanwhile, a monitoring group said that civilians have made up 42 per cent of the casualties from Russia's year-long air campaign in Syria.
The Russian airstrikes have killed 2,758 Islamic State jihadists, 2,851 rebel and Fatah al-Sham Front (formerly al-Nusra Front) fighters, and 3,915 civilians, the Syrian Observatory for Human Rights said.
The observatory reported fierce fighting between two of the most powerful rebel groups in northern Syria.
Hardline Islamist force Ahrar al-Sham drove al-Qaeda-linked Jund al-Aqsa from several of its positions, the observatory said.
Ahrar al-Sham, a key ally of Fatah al-Sham, is thought to be the largest rebel force in northern Syria.
Jund al-Aqsa is spearheading a rebel campaign that has driven back government forces north of the key central city of Hama in recent weeks.
Russia's lower house of parliament on Friday ratified an agreement presented by President Vladimir Putin to indefinitely station combat aircraft and soldiers in Syria.
The deal will bring Syria closer to peace, the legislature's deputy speaker, Sergei Zheleznyak, said in comments carried by Russian state news agency TASS.
The deal was created last year by Moscow and Damascus and stipulates impunity for Russian servicemen and the ability to freely transport weaponry and munitions in Syria.
Russia has two military bases in coastal Syria, the Khmeimim airbase in the province of Latakia and a naval facility in the port city of Tartus.
By Peter Voskamp and Peter Spinella
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