Russian Foreign Minister Sergei Lavrov made the call in a telephone conversation with US Secretary of State John Kerry on Wednesday.
Russia’s Foreign Ministry said that during the conversation, Lavrov touched on the latest revelations that the Jabhat Fateh al-Sham terrorist group, al-Qaeda's Syrian branch formerly known as al-Nusra Front, has received foreign support, including supplies of US weapons.
In an interview with German-language daily Kölner Stadt-Anzeiger earlier this week, a Fateh al-Sham commander, identified only as Abu al-Ezz, confirmed that the US is supporting the terror group, saying, “The Americans are on our side.”
He further noted that the group has been receiving funding from Saudi Arabia, Qatar and Kuwait, and has also obtained tanks and artillery from Libya via Turkey.
Elsewhere in his remarks, Lavrov said that several opposition groups in Syria, which are said to be moderate, have taken advantage of the latest Syria truce to join other terrorist outfits and continue fighting against the government forces.
"Once again, Lavrov drew attention to the fact that a number of anti-government units, which Washington calls ‘moderate’, had not only refused to stick to the September 9 Russian-US agreement for strengthening the ceasefire and providing humanitarian access, but were instead merging with Jabhat Fateh al-Sham and continued military operations against the Syrian army side by side with those al-Qaeda units," the Russian Foreign Ministry added.
The Moscow-Washington brokered ceasefire in Syria came to an end on September 19 after one week.
Damascus refused to extend the deal after deadly US-led air raids on its military base near the eastern city of Dayr al-Zawr in violation of the agreement.
Also during Wednesday’s phone call, Kerry threatened to halt its diplomacy with Russia on Syria, claiming that Moscow was accountable for an escalation of fighting in the northwestern Syrian city of Aleppo.
US State Department spokesman John Kirby said in a statement that Kerry underlined his country's readiness to "suspend US-Russia bilateral engagement on Syria," including talks on a possible counter-extremist partnership, "unless Russia takes immediate steps” to restore the ceasefire.
Since March 2011, Syria has been gripped by militancy it blames on some Western states and their regional allies.