US special envoy for Syria engagement and the fight against the Islamic State group, James Jeffrey, said Washington is "very, very worried" about the Syrian regime's assault - backed by Russia - on Idlib province.
Jeffrey called on Moscow to help halt the bloody assault that has uprooted hundreds of thousands of civilians and killed hundreds.
"This is a dangerous conflict. It needs to be brought to an end. Russia needs to change its policies," Jeffrey said.
This comes as the Syrian regime and Russian forces on Wednesday launched new attacks aimed at capturing Idlib City.
Syrian opposition military sources said Arabic-language service that Russian planes were bombing the road between Idlib City and the village of Qumenas, six kilometres southeast of the provincial capital.
Regime forces on Tuesday advanced to within eight kilometres of the city, having captured the village of Nayrab one day after opposition forces drove them out.
Regime military sources said that they were trying to advance from Nayrab to Qumenas amid heavy fighting with the opposition.
The sharp escalation of violence prompted the US, UK and France called for an urgent UN Security Council meeting on Thursday to discuss the situation in Idlib.
"We're seeing not just the Russians but Iranians and Hezbollah actively involved in supporting the Syrian offensive. We don't know whether the offensive is just to get to the M4-M5 road, or it may continue further," Jeffrey said, in reference to the strategic highways connecting Syria's Aleppo to Hama and Latakia on the Mediterranean coast and the other linking Aleppo and Damascus.
Jeffrey said Moscow could change its policies and meet the requirements of the international community without ousting Assad. "Those requirements are not unreasonable. ... They require a change in that (Assad) government's behaviour. That government would not survive a week without the Russian assistance."
Moscow's presence complicates matters. Following the partial withdrawal of US troops from northern Syria, Moscow has increased its own military presence in the northeast and had entered into de-confliction agreements between the two sides.
But Jeffrey said on Wednesday there have been more incidents of Russia's violating the terms of de-conflict agreement in what he characterised as an attempt to challenge the US presence in there.
"We've seen a limited number of occasions where... they've tried to come deep into the area where we and the [Syrian Democratic Forces] are patrolling well inside the basic lines we have sketched," he said.
"Those are the ones that are worrying me."
He said that while the numbers of such incidents were not very high, they were on the rise.
"Thus is troubling," he said, and called on Moscow to adhere fully to the de-conflict agreements made with the US.
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