World powers meeting to discuss Syria have agreed to implement a nationwide "cessation of hostilities", US Secretary of State John Kerry says.
But he said the ceasefire would not apply to the fight with jihadist groups Islamic State (IS) and al-Nusra Front.
He also said the powers had agreed to accelerate and expand the delivery of humanitarian aid.
More than 250,000 people have died and 11 million have fled their homes in almost five years of war in Syria.
The announcement came as the Syrian army, backed by Russian air strikes, advanced in Aleppo province.
A United Nations task force will be set up to ensure humanitarian access is granted to all sides, Mr Kerry said.
Mr Kerry admitted the ceasefire plan was "ambitious" and said the real test would be whether the parties on the ground honoured the commitments.
He made the announcement alongside his Russian counterpart Sergei Lavrov and the UN special envoy for Syria, Staffan de Mistura.
Mr Lavrov said there were "reasons to hope we have done a great job today".
At the press conference Mr Kerry again suggested that Russian strikes were targeting opposition forces, rather than terrorists as Moscow says.
But both men agreed that peace talks involving the Syrian government and rebels should resume as soon as possible.