The United States expressed concern on Tuesday at what it said was the heavy civilian toll of "indiscriminate" Russian air strikes in Syria.

US Secretary of State John Kerry spoke to his Russian counterpart Foreign Minister Sergei Lavrov on Monday and conveyed Washington's worries.

State Department spokesman Mark Toner, citing reports from what he called "credible human rights organizations," said: "Russian air strikes in Syria have killed hundreds of civilians, including first responders, (and) hit medical facilities, schools and markets."

Toner added that in October and the first half of November, more than 130,000 Syrians had been forced to flee their homes, in part because of intensified Russian bombing.

Moscow has angrily denied reports from Amnesty International, Human Rights Watch and Syrian rights groups that its air campaign in support of President Bashar al-Assad's government is hitting civilians.

It insists its operation is aimed at "terrorists" and that it takes care to protect civilians, while working with the United States and the United Nations to push for a negotiated end to the war.

But Toner said the United States had expressed its concerns to Moscow about "these indiscriminate attacks ... on infrastructure, on medical facilities, on civilians."

Kerry also complained to Lavrov about Friday's killing of Syrian rebel chief Zahran Alloush, in an air strike claimed by Assad's government.

Toner said the United States had concerns about the rhetoric and tactics of Alloush's rebel group -- Jaish al-Islam -- but had noted that he was ready to take part in UN-mediated peace talks.

"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," Toner said.