World powers have agreed to seek a nationwide "cessation of hostilities" in Syria to begin in a week's time, after talks in Germany. The halt will not apply to the battle against jihadist groups Islamic State (IS) and al-Nusra Front.
Ministers from the International Syria Support Group also agreed to accelerate and expand aid deliveries.
The announcement comes as the Syrian army, backed by Russian air strikes, advances in Aleppo province.
The move threatens to encircle tens of thousands of civilians in rebel-held parts of the major city of Aleppo.
US Secretary of State John Kerry admitted the ceasefire plan was "ambitious" and said the real test would be whether the parties honoured the commitments.
"What we have here are words on paper, what we need to see in the next few days are actions on the ground," he said.
A task force chaired by the US and Russia will work to implement the truce through consultations with Syria's warring factions.
Aid deliveries for besieged Syrian communities are due to begin as early as Friday.
Mr Kerry 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". An earlier proposal from Russia envisaged a truce starting on 1 March.
At the press conference Mr Kerry again suggested that Russian strikes were targeting what the West sees as moderate opposition forces, rather than terrorists, as Moscow says.
British Foreign Secretary Philip Hammond said the cessation would only work if Russia halted its raids, although Mr Lavrov said they would continue.
The Support Group also agreed that peace talks involving the Syrian government and rebels should resume as soon as possible.
Initial talks were suspended just days after they began earlier this month in Geneva, in the wake of the Aleppo offensive.
Thousands of people displaced by the fighting have been stranded at the border with Turkey and aid agencies have warned of a rapidly deteriorating humanitarian situation.
A spokesman for the Syrian opposition, Salem al-Meslet, told the BBC "if we see action and implementation on the ground, we will be soon in Geneva".
