WASHINGTON, May 15 (Reuters) - The United States has evidence the regime of Syrian President Bashar al-Assad has built a crematorium at a large military prison outside the capital Damascus, a State Department official said on Monday.

Stuart Jones, the acting assistant secretary for Near Eastern Affairs, said U.S. officials believe the crematorium would be used to dispose of bodies at a prison, where they believe Assad's government authorized the mass hangings of thousands of inmates during Syria's 6-year-old civil war.

"Credible sources have believed that many of the bodies have been disposed in mass graves," Jones told reporters at a briefing.

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16 PHOTOS US airstrikes in Syria -- launches and impact See Gallery US airstrikes in Syria -- launches and impact U.S. Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea on April 7, 2017. (Robert S. Price/Courtesy U.S. Navy/Handout via REUTERS) Battle damage assessment image of Shayrat Airfield, Syria, is seen in this DigitalGlobe satellite image, released by the Pentagon following U.S. Tomahawk Land Attack Missile strikes from Arleigh Burke-class guided-missile destroyers, the USS Ross and USS Porter on April 7, 2017. (DigitalGlobe/Courtesy U.S. Department of Defense/Handout via REUTERS) U.S. Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea on April 7, 2017. (Robert S. Price/Courtesy U.S. Navy/Handout via REUTERS) Shayrat Airfield in Homs, Syria is seen in this DigitalGlobe satellite image released by the U.S. Defense Department on April 6, 2017 after announcing U.S. forces conducted a cruise missile strike against the Syrian Air Force airfield. DigitalGlobe/Courtesy U.S. Department of Defense/Handout via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY. NO RESALES. NO ARCHIVE. MANDATORY CREDIT. U.S. Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea on April 7, 2017. (Robert S. Price/Courtesy U.S. Navy/Handout via REUTERS) U.S. Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea on April 7, 2017. (Robert S. Price/Courtesy U.S. Navy/Handout via REUTERS) Shayrat Airfield in Homs, Syria is seen in this DigitalGlobe satellite image on February 18, 2017 and released by the U.S. Defense Department on April 6, 2017 after announcing U.S. forces conducted a cruise missile strike against the Syrian Air Force airfield. (DigitalGlobe/Courtesy U.S. Department of Defense/Handout via REUTERS) U.S. Navy guided-missile destroyer USS Porter (DDG 78) conducts strike operations while in the Mediterranean Sea which U.S. Defense Department said was a part of cruise missile strike against Syria on April 7, 2017. (Ford Williams/Courtesy U.S. Navy/Handout via REUTERS) U.S. Navy guided-missile destroyer USS Porter (DDG 78) conducts strike operations while in the Mediterranean Sea, April 7, 2017. Porter, forward-deployed to Rota, Spain, is conducting naval operations in the U.S. 6th Fleet area of operations in support of U.S. national security interests in Europe. (U.S. Navy photo by Mass Communication Specialist 3rd Class Ford Williams/Released) Battle damage assessment image of Shayrat Airfield, Syria, is seen in this DigitalGlobe satellite image, released by the Pentagon following U.S. Tomahawk Land Attack Missile strikes from Arleigh Burke-class guided-missile destroyers, the USS Ross and USS Porter on April 7, 2017. (DigitalGlobe/Courtesy U.S. Department of Defense/Handout via REUTERS) U.S. Navy guided-missile destroyer USS Porter (DDG 78) conducts strike operations while in the Mediterranean Sea which U.S. Defense Department said was a part of cruise missile strike against Syria on April 7, 2017. (Ford Williams/Courtesy U.S. Navy/Handout via REUTER) U.S. Navy guided-missile destroyer USS Porter (DDG 78) conducts strike operations while in the Mediterranean Sea which U.S. Defense Department said was a part of cruise missile strike against Syria on April 7, 2017. (Ford Williams/Courtesy U.S. Navy/Handout via REUTERS) Battle damage assessment image of Shayrat Airfield, Syria, is seen in this DigitalGlobe satellite image, released by the Pentagon following U.S. Tomahawk Land Attack Missile strikes from Arleigh Burke-class guided-missile destroyers, the USS Ross and USS Porter on April 7, 2017. (DigitalGlobe/Courtesy U.S. Department of Defense/Handout via REUTERS) U.S. Navy guided-missile destroyer USS Porter (DDG 78) conducts strike operations while in the Mediterranean Sea which U.S. Defense Department said was a part of cruise missile strike against Syria on April 7, 2017. (Ford Williams/Courtesy U.S. Navy/Handout via REUTERS) Syrian shepherds tend their flock near the damaged Shayrat ('ash-Shairat') airfield at the Syrian government forces military base targeted earlier overnight by US Tomahawk cruise missiles, southeast of the central and third largest Syrian city of Homs, on April 7, 2017. / AFP PHOTO / STRINGER (Photo credit should read STRINGER/AFP/Getty Images) A picture taken on April 7, 2017 shows a view of the damaged Shayrat ('ash-Shairat') airfield at the Syrian government forces military base targeted earlier overnight by US Tomahawk cruise missiles, southeast of the central and third largest Syrian city of Homs. / AFP PHOTO / STRINGER (Photo credit should read STRINGER/AFP/Getty Images) Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

"We now believe that the Syrian regime has installed a crematorium in the Sednaya prison complex which could dispose of detainees' remains with little evidence," he said.

The United States believes the crematorium would be used to cover up mass murders at the prison and will present the evidence to the international community, Jones said.

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Amnesty International reported in February that an average of 20 to 50 people were hanged each week at the Sednaya military prison north of Damascus. Between 5,000 and 13,000 people were executed at Sednaya in the four years since a popular uprising descended into war, it said.

Jones also was not optimistic about a deal to set up "de-escalation zones" inside Syria in an attempt to reduce violence and save lives. The deal was brokered by Russia with support from Iran and Turkey during ceasefire talks in the Kazakh capital of Astana last week.

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"In light of the failures of the past ceasefire agreements, we have reason to be skeptical," Jones said. "The (Assad) regime must stop all attacks on civilian and opposition forces. And Russia must bear responsibility to ensure regime compliance.

25 PHOTOS Syria's scars of war See Gallery Syria's scars of war Visitors walk inside Aleppo's Umayyad mosque, Syria October 6, 2010. REUTERS/Khalil Ashawi SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. Men walk near damage inside Aleppo's Umayyad mosque, Syria December 17, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows Aleppo's Umayyad mosque, Syria, before it was damaged on October 6, 2010 (top) and after it was damaged (bottom) December 17, 2016. REUTERS/Khalil Ashawi (top)/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. People walk inside Aleppo's Umayyad mosque, Syria March 12, 2009. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. Damage is seen inside Aleppo's Umayyad mosque, Syria December 13, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows Aleppo's Umayyad mosque, Syria, before it was damaged on March 12, 2009 (top) and after it was damaged (bottom) December 13, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows Shahba Mall, one of the largest commercial shopping centres in Syria, before it was damaged on December 12, 2009 (L) and after it was damaged (R) on October 16, 2014. REUTERS/Khalil Ashawi (L)/Abdalrhman Ismail SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A woman carries an umbrella in al-Sheebani school, in the Old City of Aleppo, Syria May 14, 2008. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A general view shows damage in al-Sheebani school building, in the Old City of Aleppo, Syria December 17, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows al-Sheebani school's building, in the Old City of Aleppo, Syria before it was damaged on May 14, 2008 (top) and after it was damaged (bottom) December 17, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A damaged chandelier lies on the ground at Hamam El Nahasin, in the Old City of Aleppo, Syria December 17, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows Hamam El Nahasin, in the Old City of Aleppo, Syria before it was damaged on October 6, 2010 (top) and after it was damaged December 17, 2016. REUTERS/(Top)Khalil Ashawi/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. People attend a music concert in al-Sheebani school's courtyard, in the Old City of Aleppo, Syria June 6, 2009. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A general view shows damage in al-Sheebani school's courtyard, in the Old City of Aleppo, Syria December 17, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows al-Sheebani school's courtyard, in the Old City of Aleppo, Syria before it was damaged on June 6, 2009 (top) and after it was damaged December 17, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A general view shows the entrance to al-Zarab souk in the Old city of Aleppo, Syria November 24, 2008. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A general view shows damage on the entrance to al-Zarab souk in the Old city of Aleppo, Syria December 13, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows the entrance to al-Zarab souk in the Old city of Aleppo, Syria November 24, 2008 (top) and after it was damaged December 13, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. People walk inside Aleppo's Umayyad mosque, Syria March 12, 2009. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. Men walk near damage inside Aleppo's Umayyad mosque, Syria December 13, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows Aleppo's Umayyad mosque, Syria, before it was damaged on March 12, 2009 (top) and after it was damaged December 13, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A general view shows the Old City of Aleppo, Syria November 24, 2008. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A general view shows damage in the Old City of Aleppo, Syria December 13, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows the Old City of Aleppo, Syria on November 24, 2008 (top) and after it was damaged December 13, 2016. REUTERS/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. A combination picture shows Aleppo's historic citadel, in the Old City of Aleppo, Syria before it was damaged on August 9, 2010 (top) and after it was damaged December 13, 2016. REUTERS/Sandra Auger (top)/Omar Sanadiki SEARCH "ALEPPO HERITAGE" FOR THIS STORY. SEARCH "WIDER IMAGE" FOR ALL STORIES. Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

(Reporting by Yeganeh Torbati and Doina Chiacu; Editing by Chizu Nomiyama and Bernadette Baum)

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