HAMBURG — A cease-fire agreement reached Friday between the United States and Russia is intended to quell fighting in southwest Syria and allow anti-government rebels there to focus on the Islamic State, Secretary of State Rex Tillerson said.

Tillerson announced the cease-fire as President Trump held his first meeting with Russian President Vladimir Putin here at the Group of 20 summit.

Tillerson said the agreement, if it holds, may be a blueprint for other parts of the country. “This area in the south is our first show of success. We hope we can replicate that elsewhere,” he said.

The deal marks a new level of involvement for the U.S. in trying to resolve Syria’s civil war, which pits forces loyal to Syrian President Bashar Assad and Iranian allies against rebel forces seeking to impose a brutal Islamic rule.

It’s still unclear how the agreement will be enforced.

“We have a very clear picture of who will provide the security forces, but we have a few more details to work out,” he said. Those discussions should be finalized within a week. “The talks are very active and ongoing.”

The U.S. position remains that Assad must go. “We see no long-term role for the Assad family or the Assad regime,” Tillerson said.

Tillerson has assailed Assad for attacking his own people with chemical weapons. Trump on April 6 authorized dozens of U.S. cruise missile strikes at a Syrian government air base where a chemical attack was alleged to have been launched days earlier.

Assad and Russia denied the allegations, calling them fabricated. Russia and Iran have backed Assad since the beginning of the civil war in 2011. The fighting has caused an estimated 500,000 deaths and displaced nearly half of Syria's pre-war population of 22 million people.

Jordan's Minister of State for Media Affairs, Mohammed Momani, announced the three-way agreement with the United States and Russia is set to go into effect as of Sunday, according to the Jordan News Agency Petra.

The agreement aims at permanently de-escalating the tension in southern Syria, ending hostilities, restoring stability and allowing free access of humanitarian aid, Petra reported.

A senior State Department official involved in the talks told reporters on a conference call Friday that some details of the agreement have yet to be worked out, such as how to monitor and enforce it. The official asked not to be named because the official was not authorized to speak publicly.

The Assad government is expected to have signed on to it as well as U.S.- and Jordanian-supported forces on the ground, the official said. The idea behind the cease-fire is to stop the violence and create space for a resolution that stops the conflict, the official said.

Andrew Tabler, a Syria analyst at the Washington Institute for Near East Policy, said the agreement is meant to prevent attacks by forces loyal to Assad on U.S.- and Jordanian-supported rebel forces in southeast Syria.

“The Assad regime is on the defensive. They don’t have enough forces to go on the offensive so they rely on Iranian militias,” said Tabler. “This agreement keeps those Iranian militias out of the areas adjacent the Jordanian and Israeli borders.”

The deal has been in works for a while, in negotiations in Amman with the Jordanians, Russians and others, he said.

“Some of the heaviest bombing in the last few months has been in that area by the regime,” Tabler said.

The Islamic State, also known as ISIS, is in the area covered by the cease-fire and will not be party to the agreement. ISIS is hemmed in against the Israeli-controlled Golan Heights, and under attack by U.S.-supported rebel forces.

“That’s why were supporting those rebels. This agreement should allow those rebels to focus on ISIS and for the regime not to attack rebel forces,” Tabler said.

The question now is whether Russia will be able to enforce the agreement on its Syrian and Iranian allies, he said. “That’s not certain.”

Several previous cease-fires negotiated between the Obama administration and Russia failed after Syrian government forces attacked U.S.-backed rebels, claiming they were terrorists.

The deal is separate from “de-escalation zones” that were to be created under a deal brokered by Russia, Turkey and Iran earlier this year. The U.S. was not a part of that deal. Follow-up talks this week in Astana, Kazakhstan, to finalize a cease-fire in those zones failed to reach agreement, according to the Associated Press.

Earlier in the week, Syria’s military had said it was stopping combat operations in the south of Syria for four days, in advance of a new round of Russia-sponsored talks in Astana. That move covered southern provinces of Daraa, Quneitra and Sweida. Syria’s government briefly extended that unilateral cease-fire, which is now set to expire Saturday — a day before the U.S. and Russian deal would take effect.

Frederic Hof, a former special adviser for transition in Syria under President Barack Obama, said a key variable in whether the cease-fire holds is the extent to which Iran and Assad have signed on to the arrangements agreed to by Washington and Moscow.

"We’ve seen this act before," Hof said, referring to the so-called "cessation of hostilities" negotiated between then-secretary of State John Kerry and Russia's Foreign Minister Sergey Lavrov in February 2016.

That cease-fire "held for a week to 10 days and fell apart completely," Hof said.

Al Qaeda's affiliate in Syria, the Nusra Front, has factions in southwest Syria and would not be party to the agreement, which will give the Russians and Assad the opportunity to attack those groups, Hof said.

And even if the rebels are left alone to wipe out ISIS, the underlying civil war in Syria will remain, he said.

"You can kill each and every (Islamic State) person in southwest Syria but that will not pacify the area for any period of time," Hof said. "There are too many spoilers and there is an ongoing conflict between the Assad regime and the rebels."

On the other hand, there is merit to a cease-fire, he said.

"Just a few days ago the Assad regime was conducting barrel bombs in Daraa," Hof said. "If the Russians are successful in grounding the Syrian air force there is no harm in that."

Dorell reported from Washington, D.C.

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