WASHINGTON - US Secretary of State John Kerry called his Russian counterpart Foreign Minister Sergei Lavrov on Friday to demand that Moscow press its Syrian allies to respect a crumbling ceasefire.

"Secretary Kerry said the United States expected Russia to urge the regime to comply with the cessation and that we would work with the opposition to do the same," US spokesman John Kirby said.

Kerry's call came as a new round of fierce fighting around the northern city of Aleppo overshadowed peace talks aimed at ending Syria's five-year-old civil war. According to Kirby, Kerry told Lavrov of Washington's "serious concerns over the ongoing threats to the cessation of hostilities in Syria and the urgent need for the Assad regime to stop its violations of the cessation."

US officials have complained that Russian jets appear to be flying in support of Syrian forces attacking rebel positions in Aleppo, despite having signed on to efforts to promote a political settlement. But the situation has been complicated by the presence of fighters from the Al-Qaeda linked Al-Nusra Front in Aleppo. Nusra is not party to the ceasefire and both Moscow and Damascus reserve the right to strike "terrorists."