Lt. Gen. Sergei Rudskoi of the Russian military’s General Staff speaks at a briefing in Moscow, Russia, on Monday, April 11, 2016.

The United States expressed concern over threats to the ceasefire in Syria on Monday, amid reports that Russian-backed government troops are planning an offensive.

Fighting has increased around the northern city of Aleppo, where a variety of rebel factions are fighting forces loyal to President Bashar al-Assad.

On Sunday, Syria’s Prime Minister Wael al-Halqi warned that the regime and its “Russian partners” were readying an offensive to recapture the city.

In Washington, State Department spokesman Mark Toner said Secretary of State John Kerry had called Russian Foreign Minister Sergei Lavrov on Sunday to express concern.

“We are very, very concerned about the recent increase in violence. And that includes actions we believe are in contravention to the cessation of hostilities,” Toner said.

Although the February 27 ceasefire deal has largely held, hardline groups such as ISIS, also referred to as Daesh, and the Al-Qaeda linked Al-Nusra Front are seen as fair game.

Aleppo is home to a variety of rebel forces, however, and Washington is concerned that any Russian-backed assault on Al-Nusra may spread to also target moderate factions.

This in turn could cause the ceasefire to collapse and even derail the UN-mediated political peace process due to resume in Geneva on Wednesday.

“One of the things the secretary stressed very strongly in his phone call yesterday with Foreign Minister Lavrov, is that we need to make certain that we work to determine which fighting group is where,” Toner said.

“We’ve talked about the fact that everybody needs to focus on Nusra and Daesh, but we can’t have overlap and we can’t have violations against those groups who have bought into the ceasefire or the cessation.”

Related articles across the web

• Russia Says Special Forces Unit Operating In Syria
• Syria, Turkey should remain good neighbors - Assad
• Syrian Opposition Needs to Pursue Realistic Goals In Peace Talks
• Syrian army says close to capturing ISIL-held Palmyra
• UN envoy pressures Syria regime on transition
• Kerry in Moscow to sound out Putin on Assad rule
• Kerry seeks answers on Assad's future in Kremlin talks
• Damascus can't take Russian support for granted: Analyst
• Russian warplanes deliver airstrikes at 146 facilities near Palmyra - defense ministry
• Syria analyst: Damascus cannot take Russian support for granted