World powers gathered in Munich, Germany, have agreed to seek a nationwide “cessation of hostilities” in Syria to begin in a week’s time.
The halt will not apply to the battle against jihadist groups ISIS and al-Nusra Front.
The 17-member International Syria Support Group (ISSG) 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.
The Syrian government has not yet responded, though a key rebel coalition welcomed the announcement.
More than 250,000 people have been killed and 13.5 million displaced in almost five years of fighting in Syria.
Secretary of State John Kerry admitted the ceasefire plan was “ambitious” and said the real test would be whether the various parties honored 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 rival groups.
Aid deliveries for besieged Syrian communities are due to begin as early as February 12.
Hohn Kerry made the announcement alongside his Russian counterpart Sergei Lavrov and the UN special envoy for Syria, Staffan de Mistura.
Sergei Lavrov said there were “reasons to hope we have done a great job today”. An earlier proposal from Russia envisaged a truce starting on March 1.
At the news conference John Kerry again suggested that Russian strikes were targeting what the West sees as moderate opposition forces, rather than terrorists, as Moscow says.
The ISSG 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.
