US Secretary of State John Kerry said diplomats meeting in Munich have agreed to work with Syrian parties to implement a "nationwide cessation of hostilities" early on Friday.
Kerry said that the target is to implement the ceasefire in a week's time. He said that would not apply to the Islamic State group (IS) and the extremist al-Nusra Front.
Kerry said diplomats from the US, Russia and other powers also agreed to "accelerate and expand" the immediate delivery of humanitarian aid, bringing aid to besieged areas.
A working group is to start meeting in Geneva Immediately to oversee the agreement.
Salem al-Meslet, spokesman for the High Negotiating Commission (HNC) cautiously welcomed the announcement.
"We welcome the efforts our friends making to bring relief to the Syrian people - and it must be all Syrians. We must see action on the ground and if we see action and implementation we will see you soon in Geneva," said al-Meslet.
Kerry's announcement comes as Syrian regime troops and allied militias backed by Russian airstrikes advanced on the northern city of Aleppo, causing a mass exodus of residents.
The Syrian Observatory for Human Rights has meanwhile estimated thatat least 500 people have been killed since the offensive on Aleppo began.