Russia came under renewed criticism on December 29 for what the United States said is "indiscriminate" bombing in Syria that is killing civilians and endangering peace talks.
U.S. State Department spokesman Mark Toner cited recent "extremely disturbing" reports by nongovernmental groups describing Russian air strikes that killed hundreds of civilians, including emergency response workers, and hit medical facilities, schools, and markets.
U.S. Secretary of State John Kerry has raised concerns about these "indiscriminate attacks" with Russian Foreign Minister Sergei Lavrov, he said.
"We've seen a marked and troubling increase in reports of these civilian casualties since Russia commenced its air campaign," Toner said.
Kerry also complained to Lavrov about an air strike last week that killed Zahran Alloush, a prominent Syrian rebel leader who had supported the peace talks, Toner said.
"Attacks on those who could be part of this political process, as well as attacks that kill innocent civilians, undermine efforts to find a political resolution," he said.
"It is our hope that it does not send a discouraging message to other members of the Syrian opposition...who have expressed a willingness to take part in this process," he said.
Negotiations between representatives of Syrian President Bashar al-Assad's government and the opposition were expected to start next month, though hurdles remain.
Saudi Arabia also expressed consternation about the Alloush killing, which came after Riyadh had forged an agreement with the Syrian leader's powerful Army of Islam group and more than 100 other Syrian opposition groups to start peace talks with Assad.
"I don't know what the Russians have in mind," Saudi Foreign Minister Adel al-Jubeir said at a news conference in Riyadh December 29. "If we want to reach a peaceful solution in Syria, we must deal with all the Syrian groups whose hands are not smeared with terrorism."
Russia and Syria contend that Alloush's Army of Islam group is a terrorist organization, however.
Besides causing hundreds of deaths, since the Russian air strikes started in September more than 130,000 Syrians have been forced to flee their homes, Toner said.
Moscow has not commented on the Alloush killing, but it has angrily denied reports from Amnesty International, Human Rights Watch, and Syrian rights groups that its air campaign is hitting civilians.
It insists the campaign is aimed at "terrorists" and that it takes care to protect civilians.
The United States and other Western countries maintain that the Russian strikes have mostly struck Syrian rebel targets other than the Islamic State group which Russia claims to be targeting.