James Jeffrey, the US envoy for Syria, said on Thursday that Russia had enough leverage to bring the Assad regime to the negotiating table and push Iran-led forces out of the country.
Mr Jeffrey, the White House point man for the global coalition against ISIS in Syria, spoke before a trip to Geneva next week.
He is due to discuss the political process in Syria, especially progress on the constitutional committee, with regional, European and Russian envoys.
Mr Jeffrey said he considered Moscow’s role as instrumental in getting a settlement to the conflict.
“Iran was much involved in Syria up to 2015 and, even with Iranian help, [President Bashar Al] Assad could not stop the opposition gaining ever more territory,” he said.
“It was the entry of Russian forces, specifically air forces, into the conflict in late 2015 that turned the tide. That is still the situation."
Mr Jeffrey said Tehran lacked the resources, ground forces and air power to win a military victory.
The continuing survival of the Assad regime was “totally the work of Russia”, and only Moscow could bring the Syrian leader to the negotiating table.
Mr Al Assad’s decision to finally allow to the constitutional committee to meet was a sign that it was already bending to Russian pressure, he said.
“It is sign that the Russians have been effectively using persuasion, pressure, whatever you would call it," Mr Jeffrey said. "We urge them to use more.”
But he said Russia had not yet made a decision “to shift totally from a military solution option to a political solution”.
Mr Jeffrey said the Assad regime “must accept the will of the Syrian people to live in peace and not be threatened by targeted violence, arbitrary detentions, starvation, brutality, chemical weapons attacks and everything else they’re doing”.
He said he hoped the Geneva process could bring about a new constitution and “a free and fair election under UN supervision involving all Syrians, not the phony election we just saw Assad run for his Parliament”.
Mr Jeffrey pledged to continue economic and political pressure on the Assad regime.
On Wednesday, Mr Al Assad appeared to feel the economic pinch.
He told Syrians to support their currency, which has lost almost two thirds of its value since the start of the war.
“Our support of the pound is a source of our strength,” he said.
Through the Caesar Act, the US imposed two rounds of sanctions on the regime in June and July, against Mr Al Assad's wife Asma and eldest son Hafez, among other key figures in the security and political apparatus.
A mask-clad civil society volunteer marches with an effigy depicting the SARS-CoV-2 virion, the agent responsible for the COVID-19 coronavirus disease, during an awareness campaign about the novel coronavirus pandemic, urging people to remain at home, in Syria's northwestern city of Idlib in Idlib province on March 24, 2020. (Photo by Abdulaziz KETAZ / AFP)
Syrian Kurdish passengers who were stranded in Damascus arrive in Qamishli in Syria's northeastern Hasakeh province on April 5, 2020, after being stranded in Damascus for the past weeks. (Photo by DELIL SOULEIMAN / AFP)
A drone image taken on April 9, 2020, shows a sanitation worker disinfecting a camp for displaced Syrians next to the Idlib municipal stadium in the northwestern Syrian city, during a campaign to limit the spread of the coronavirus (COVID-19) pandemic. (Photo by OMAR HAJ KADOUR / AFP)
A medic checks the body temperature of young passengers, as a preventive measure against the coronavirus, upon their arrival by bus in Syria's Kurdish area from Iraqi Kurdistan via the Semalka border crossing in northeastern Syria on February 26, 2020. (Photo by Delil SOULEIMAN / AFP)
A picture taken on April 27, 2020 shows Syrians who returned from Turkey standing at a quarantine facility in the countryside of the town of Jisr al-Shughur, west of the mostly rebel-held Syrian province of Idlib, on April 27, 2020 during the coronavirus COVID-19 pandemic. / AFP / Abdulaziz KETAZ
epa08392436 A truck for prevention against the COVID-19 coronavirus disease, operated by local NGO 'Violet Organization', drives through the streets of Idlib, Syria, 29 April 2020. EPA/YAHYA NEMAH
Artist Aziz al-Asmar paints a mural wishing for the well-being of German Chancellor Angela Merkel, who is in quarantine after being treated by a doctor who tested positive for coronavirus (COVID-19), inside a damaged building in the town of Binnish in Syria's northwestern Idlib province on March 24, 2020. (Photo by Muhammad HAJ KADOUR / AFP)
A member of the Kurdish Internal Security Forces of Asayesh urges children to return home, in Syria's northeastern city of Hasakeh on April 30, 2020, following measures taken by the Kurdish-led local authorities there, to limit the spread of the novel coronavirus. / AFP / DELIL SOULEIMAN
Syrian boys pose for a picture during an awareness workshop on Coronavirus (COVID-19) held by Doctor Ali Ghazal at a camp for displaced people in Atme town in Syria's northwestern Idlib province, near the border with Turkey, on March 14, 2020. (Photo by AAREF WATAD / AFP)
A handout picture released by the official Syrian Arab News Agency (SANA) on March 20, 2020 shows Syrian Red Crescent vehicles spraying disinfectant along a street in the capital Damascus, as part of measures against the spread of COVID-19 coronavirus disease. (Photo by - / SANA / AFP) / == RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / HO / SANA" - NO MARKETING NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS ==
A volunteer from the Violet organisation disinfects a mosque in Syria's northwestern city of Idlib on April 25, 2020, from coronavirus (COVID-19) during the Muslim holy fasting month of Ramadan. (Photo by OMAR HAJ KADOUR / AFP)
Only a few people walk in the century-old covered bazaar of Hamidiya in Syria's capital Damascus on March 24, 2020, after measures were taken by the authorities to fight the novel coronavirus pandemic. - Across much of the Syrian capital, with squares and markets once thronging with people even during the war, are now almost entirely empty. Five cases of COVID-19 have been reported in the country since Sunday, and the authorities have ordered all non-essential businesses closed. (Photo by LOUAI BESHARA / AFP)
Researchers are examining the possibility of using inhalers to introduce stem cells into a patients lungs. AFP
A member of the Syrian Violet NGO disinfects a triage tent erected for suspected coronavirus patients outside the Ibn Sina Hospital in Syria's northwestern Idlib city on March 19, 2020. - Syrian authorities on March 13 announced measures aimed at preventing coronavirus from reaching the war-torn country, including school closures and a ban on smoking shisha in cafes, state media reported. (Photo by Abdulaziz KETAZ / AFP)
A young pupil follows a lesson on a mobile telephone inside a tent, in a camp for displaced Syrians in the village of Kafr Yahmoul in the northwestern Idlib province, amid the coronavirus pandemic on April 3, 2020. - Like in much of the world, educators in Syria are taking classes online after the country's various regions sent pupils home hoping to stem the COVID-19 pandemic. But distance learning is no small feat in a country battered by nine years of war, where fighting has displaced millions and the electricity supply is sporadic at best. (Photo by Aref TAMMAWI / AFP)
Updated: August 14, 2020 04:49 AM