The United States criticized Russia on Tuesday for killing hundreds of civilians in airstrikes in Syria and accused Moscow of undermining hopes for a cease-fire between Bashar Assad’s government and leading rebel groups.

The surprisingly sharp critique came as Washington banked on Moscow’s help to launch a Syrian peace process that would allow both countries to focus on defeating the Islamic State. Negotiations between representatives of Assad’s government and the opposition were expected to start next month, though hurdles remain.

In the meantime, the Arab country’s almost five-year civil war raged on. Human rights groups say Russian airstrikes are contributing to a growing humanitarian crisis that includes more than a quarter-million people killed, millions left homeless or as refugees, and vast expanses of chaos that the Islamic States has exploited to carve out a repressive, self-proclaimed caliphate.

Amid reports of indiscriminate killing by Russia, including the use of cluster bombs, Secretary of State John Kerry called Russian Foreign Minister Sergey Lavrov earlier this week to share his concerns. U.S. officials wouldn’t describe Lavrov’s response.

“The reports of Russian attacks on Syrian civilians are extremely disturbing,” State Department spokesman Mark Toner told reporters Tuesday. Activist accounts suggest Russian strikes “killed hundreds of civilians, including first responders” and “hit medical facilities, schools and markets,” he said.

The campaign uprooted some 130,000 Syrians in October and the first half of November alone, Toner said.

An Amnesty International report last week cited evidence of Russian use of cluster munitions and unguided bombs in populated residential areas. The group denounced what it called Russia’s “shameful failure” to acknowledge civilian killings.

Russia immediately rejected the claims, calling them “clichés and fakes.”

While Toner didn’t endorse all of the findings directly, he said the U.S. has “seen a marked and troubling increase in reports of civilian casualties since Russia commenced its air campaign there” in late September.

He repeated Western assessments that most of Moscow’s bombs haven’t targeted Islamic State and other terrorist fighters, as the Kremlin argues. Instead, he said the “large majority” have struck areas held by Assad’s opponents, where many have been killed or wounded.

Those attacks may include one last weekend that killed Zahran Alloush, a top Syrian rebel commander who led one of the most powerful groups battling Assad’s forces. Although the U.S. has steered clear of Alloush’s Army of Islam, it had praised the hard-line conservative group for supporting the U.S.-Russian mediation effort with Assad and opposing the Islamic State. Syria’s army said it was responsible for the strike, though opposition figures blamed Russia.

In Washington’s view, such killings may prove the most damaging in prolonging the conflict. If top rebels can’t be assured that they and their fighters will be protected after laying down their arms, they’ll have little motivation to join a peace process that could keep Assad in power for months, if not years, to come.

And if Russia was indeed responsible for Alloush’s death, the recent cooperation between Washington and Moscow to stop Syria’s bloodshed may prove untenable.

Toner didn’t ascribe blame for Alloush’s death, but said Kerry discussed the attack with Lavrov.

The U.S. is focused on “beginning a credible political process that can lead, finally, to an end to the violence in Syria and a new political path forward for the Syrian people,” 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.”

Russian officials repeatedly have rejected accusations of civilian killings, and residents and opposition activists inside Syria acknowledge they have no way to categorically differentiate Russian-operated planes from Syrian-operated planes. Amnesty’s report came days after Human Rights Watch issued similar accusations against Russia.

The talks between Syria’s government and rebels are supposed to start in Geneva in the final week of January, but several obstacles remain. The opposition must first announce its delegation to the negotiations. And a host of rival countries including the U.S., Russia, Saudi Arabia and Iran still must agree on a list of rebel militias that would be welcomed into a political process, and a separate list of terrorist groups that would be fought by all.

(AP)