The United States is "very concerned" about reports of a Russian-backed Syrian government offensive near the country's second city Aleppo, a senior administration official in Washington told AFP on Thursday.
"Such actions could well violate the cessation of hostilities, which has held for nearly seven weeks but has been under increasing pressure in recent weeks," the official said, reiterating a call for Russia to "cease any provocative actions in Syria."
Syrian government fighters, rebels and jihadists are battling for control of swathes of Aleppo province.
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Regime loyalists backed by Russia's air power pressed a fierce offensive just north of the provincial capital of Aleppo city, said the Syrian Observatory for Human Rights, straining the landmark ceasefire and a nascent round of peace talks in Geneva.
US officials "expressed our strong concerns" to all members of the 17-nation International Syria Support Group (ISSG), including Moscow, the Washington official said, urging the Kremlin to lean on Syrian President and ally Bashar al-Assad to commit to talks about a political transition.
"Now is the time for all sides to follow through on their commitment to the cessation of hostilities and to refrain from any further destabilizing actions," the official added.
In Geneva, UN envoy Staffan de Mistura criticized Assad's government for hindering efforts to deliver life-saving assistance to Syrians in need.
The United States and Russia co-chair the ISSG, seeking to achieve a peaceful settlement of the Syrian crisis.