WASHINGTON • The United States, Russia and Jordan have agreed to foster a ceasefire in a limited area of south-western Syria that will begin at noon today.

The agreement, announced by US Secretary of State Rex Tillerson after the first face-to-face meeting between President Donald Trump and Russian President Vladimir Putin, came after months of negotiations among the three countries.

Important pieces of the deal remained to be hammered out in the coming days, including who would monitor and enforce the pause in violence, said a senior State Department official who was involved in the talks.

The agreement hinges on a boundary line - as set by the US, Russia and Jordan - between areas of control for the warring forces and state proxies, the State Department official said.

But outlawed factions - including Al-Qaeda - could refuse to abide by the agreement and even actively work to undermine it, the official said.

Moscow has assured the US that the government of Syrian President Bashar al-Assad will abide by the agreement, the official said.

The assent of the government in Damascus came even though the US continues to insist that neither Mr Assad, nor any member of his family, can have a long-term role in the country's leadership.

The US and Russia agreed to similar ceasefires last year, all of which quickly disintegrated.

But Mr Tillerson said there were reasons to believe this latest attempt would not only hold, but could serve as a model for ceasefires elsewhere in Syria.

"I think this is our first indication of the US and Russia being able to work together in Syria," Mr Tillerson told reporters last Friday in Hamburg, Germany, after the meeting between Mr Trump and Mr Putin, which lasted over two hours.

"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 and violence once we defeat ISIS, and to work together towards a political process that will secure the future of the Syrian people."

He said the defeat of the Islamic State in Iraq and Syria (ISIS) was within sight, and negotiators were discussing what comes next.

He also said war fatigue - after six years of conflict - was helping to speed up potential agreements.

"In many respects, people are getting tired. They're getting weary of the conflict," Mr Tillerson said.

Russian Foreign Minister Sergey Lavrov said the ceasefire will take effect in the areas of Daraa, Quneitra and Sweida in Syria along the Jordanian border.

"At first, security around this de-escalation zone will be guaranteed by the forces and means of the Russian military police, in coordination with the Americans and Jordanians," he said.

But the senior US State Department official disputed that, saying the parties had yet to agree on monitoring.

The official said the ceasefire was, at best, a modest advance in ending the conflict.

Still, south-western Syria has experienced far less violence than in other parts of the country during the war, so the areas the ceasefire will cover is by no means at the heart of the conflict.

Mr Tillerson said Mr Trump and Mr Putin spent much of their meeting discussing the Syrian conflict.

"And I would tell you that, by and large, our objectives are exactly the same.

"How we get there, we each have a view," Mr Tillerson said.

"But there's a lot more commonality to that than there are differences. So we want to build on the commonality, and we spent a lot of time talking about next steps."

Mr Tillerson also conceded that the US may not have all the answers.

"Maybe they've got the right approach and we've got the wrong approach," he said in an extraordinary concession, considering previous statements by Mr Trump that Russia likely had prior knowledge of a Syrian chemical attack on its own people.

NYTIMES