By Jeff Mason and Denis Dyomkin
HAMBURG (Reuters) - The United States, Russia and Jordan reached a ceasefire and "de-escalation agreement" for southwestern Syria on Friday, as the U.S. government under President Donald Trump made its first attempt at peacemaking in the country's six-year-old civil war.
The ceasefire, due to start at noon Damascus time (0900 GMT) on Sunday, was announced after a meeting between Trump and Russian President Vladimir Putin at the G20 summit of major economies in the German city of Hamburg.
U.S. Secretary of State Rex Tillerson said the area covered by the ceasefire affects Jordan's security and is a "very complicated part of the Syrian battlefield."
Russia and Iran are the main international backers of Syrian President Bashar al-Assad while Washington supports some of the rebel groups fighting to topple him.
"I think this is our first indication of the U.S. 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," Tillerson said.
Previous similar ceasefires have failed to hold for long and it was not clear how much the actual combatants -- Assad's government and the main Syrian rebel forces in the southwest -- are committed to this latest effort.
Former U.S. President Barack Obama struggled to find a strategy to end Syria's civil war, which killed nearly half a million people, turned cities into ruins and forced millions to flee abroad.
Syria has also tripped up Trump, who promised better relations with Moscow but angered Russia in April by ordering missile strikes against a Syrian air base to punish Assad after a chemical weapons attack.
The Syria deal appeared to give Trump a diplomatic achievement at his first meeting with Putin where they also discussed the thorny issues of Moscow's alleged interference in the U.S. 2016 presidential election and North Korea's nuclear ambitions.
GOALS IN SYRIA
Backed by Russian air power, Assad has regained ground in the last year or so lost to the mostly Sunni Muslim rebels.
Russian Foreign Minister Sergei Lavrov said the accord includes "securing humanitarian access and setting up contacts between the opposition in the region and a monitoring centre that is being established in Jordan's capital."
The ceasefire should pave the way toward a more robust pacification effort, said a senior State Department official involved in the talks. "It is a first step in what we envision to be a more complex and robust ceasefire arrangement and de-escalation arrangement in southwest Syria, certainly more complex than ones we have tried in the past."
The official said further discussions would be needed to decide crucial aspects of the ceasefire, however, including monitoring its enforcement.
Tillerson said that by and large the objectives of the United States and Russia in Syria "are exactly the same."
But Washington and Moscow have long been at odds over Syria.
The United States has often called for the removal of Assad, who it blames for shootings of protesters at the start of the conflict and, more recently, chemical weapons attacks on civilians.
Russia and Iran strongly back the Syrian leader, who gives both countries a strategic foothold in the Mediterranean Sea.
Despite the ceasefire deal, Tillerson said the United States still sees "no long-term role for the Assad family or the Assad regime. And we have made this clear to everyone. We certainly made it clear in our discussions with Russia."
Robert Ford, who resigned in 2014 as U.S. ambassador to Syria over policy disagreements, said the Trump administration, like that of Obama, has "no national objective for the future of Syria nor any strategy for how to secure an objective were one identified."
By contrast, Russia's overall aim is clearer, said Ford, now a fellow at the Middle East Institute think tank in Washington.
"The Russian objective is to insulate Damascus and the Syrian national government from outside pressure trying to pressure it into major concessions," he said.
A group of Syrian rebels that took part in the latest peace talks in Kazakhstan this month said in a statement it had "great concern over the secret meetings between Russia and Jordan and America to conclude an individual deal for southern Syria in isolation from the north," which it described as an unprecedented event that "divides Syria and the opposition."
The Syrian government and the Southern Front, the main grouping of Western-backed rebel groups in southwest Syria, did not immediately react to the ceasefire deal.
It was not immediately clear exactly which areas of southwestern Syria would be covered by the ceasefire but earlier talks between the United States and Russia about a "de-escalation zone" covered Deraa province, on the border with Jordan, and Quneitra, which borders the Israeli-occupied Golan Heights.
British Defence Secretary Michael Fallon welcomed any ceasefire in Syria but wanted to see results on the ground.
"The recent history of the Syrian civil war is littered with ceasefires and it would be nice ... one day to have a ceasefire," Fallon said at an event in Washington.
(Additional reporting by Yara Bayoumy, Idrees Ali and Tim Ahmann and Doina Chiacu in Washington and Ellen Francis in Beirut; Writing by Alistair Bell; Editing by Yara Bayoumy and James Dalgleish)