The plan was announced Friday at the G20 summit in Hamburg, Germany, after a meeting between US President Donald Trump and his Russian counterpart, Vladimir Putin.
The leaders also agreed to establish three de-escalation zones in Syria. Russian military police, coordinating with the US and Jordan, will initially ensure security around the de-escalation zone, officials said.
The ceasefire affects the regions of Deraa and Suweida, along the Jordanian border, as well as Quneitra, near borders with Lebanon and the Israeli-occupied Golan Heights
. The areas in southern Syria will be the first in which the US is party to a de-escalation zone.
White House National Security Adviser H.R. McMaster said in a statement Saturday that such de-escalation zones are a US priority.
"The United States remains committed to defeating ISIS, helping to end the conflict in Syria, reducing suffering, and enabling people to return to their homes," he said. "This agreement is an important step toward these common goals."
The new brokered agreement is the fifth attempt at a ceasefire in a country that has been embroiled in conflict since 2011. Three of those ceasefires unraveled within months. The fourth attempt this May was never enforced when rebel groups denounced the deal shortly after its signing.
Greater US-Russia cooperation?
The US and Russia "promised to ensure that all groups there comply with the ceasefire" and "provide humanitarian access," Lavrov said.
A State Department official said Friday that they're still working on how to monitor and enforce the ceasefire.
US Secretary of State Rex Tillerson, who sat in on the discussion between the two leaders, told reporters Friday that the agreement signaled further cooperation in Syria.
The US and Russian governments have diverging long-term goals in Syria. Russia has spent the last two years supporting the Assad regime with airstrikes and special forces on the ground. In that time, the regime has gradually reasserted control over major cities, with rebel groups being finally driven out of Aleppo, the largest city in Syria, at the beginning of this year.
The US has focused on defeating ISIS by nurturing the Syrian Democratic Forces, an alliance of Arab tribes and Kurds, in the north and east of the country. Gradually, they have pushed ISIS out of large swathes of territory, aided by hundreds of coalition airstrikes, and are now inside Raqqa -- the self-proclaimed ISIS capital.
"This is our first indication of the US and Russia being able to work together in Syria," Russia's Foreign Minister Sergey Lavrov said.
News of a ceasefire agreement comes as US-backed Syrian Democratic Forces (SDF) are pushing into Raqqa.
Earlier, Tillerson said the US was prepared to work with Russia to establish no-fly zones in Syria in an effort to bring stability to the country.
A warning from Netanyahu
Israeli Prime Minister Benjamin Netanyahu gave his guarded support to the ceasefire agreement, raising the specter of an empowered Hezbollah should the de-escalation zones allow the Syrian regime to capture more territory.
The newly established de-escalation zones lie only a few kilometers from Israel's northeastern borders.
"Israel will welcome a genuine ceasefire in Syria, but this ceasefire must not enable the establishment of a military presence by Iran and its proxies in Syria in general and in southern Syria in particular," Netanyahu said ahead of his weekly Cabinet meeting on Sunday.
"I had deep discussions about this last week with US Secretary of State Tillerson and with Russian Vladimir Putin. Both told me that they understand Israel's position and will take our demands into account."
Netanyahu said fears that the threat posed by Iran-backed Hezbollah to Israel could intensify as Syrian regime forces continue to recapture territory.
"For our part, we will continue to monitor developments beyond our borders while strongly upholding our red lines: prevent the strengthening of Hezbollah via Syria, with emphasis on the acquisition of precision weapons; prevent Hezbollah -- or Iranian forces -- from establishing a ground presence along our border; and prevent the establishment of an Iranian military presence in Syria as a whole," he said.