The US, Russia and Jordan have agreed to put in place a ceasefire across south-western Syria, which is due to begin on Sunday.
Jordan has said it will take place along a line agreed by both Syrian government forces and rebels.
The ceasefire was announced after the US and Russian leaders met in person for the first time.
During their meeting Donald Trump and Vladimir Putin discussed a wide range of topics - including the war in Syria.
Their face to face was held on the sidelines of the G20 summit in the German city of Hamburg.
More than 300,000 people have lost their lives in the Syrian war, which began with anti-government protests in 2011, with another 5.5 million people having left the country and 6.3 million internally displaced, according to UN's refugee agency.
Russia and the US have backed opposing sides, with Moscow supporting the Syrian regime with its armed forces while Washington has called for the removal of Syrian President Bashar al-Assad.
This agreement, which Russian Foreign Minister Sergei Lavrov said would cover the regions of Daraa, Quneitra and Sweida, is reported to be the result of several months of undisclosed meetings between Russia and the US on Syria.
Mr Lavrov said Russia and the USA would coordinate with Jordan to act "as guarantors of the observance of this [ceasefire] by all groups".
Speaking after the meeting in Hamburg, US Secretary of State Rex Tillerson said: "I think this is our first indication of the US and Russia being able to work together in Syria, and as a result of that we had a very lengthy discussion regarding other areas in Syria that we can continue to work together on to de-escalate the areas."
Mr Tillerson said the meeting also showed the two countries eventual aims for Syria "are exactly the same" - but they differed on how they should be achieved.