BEIRUT, May 26 (Reuters) - Air strikes since Thursday evening have killed more than 100 people including children and other family members of Islamic State fighters in al-Mayadin, a town held by the jihadists near Deir al-Zor in eastern Syria, a war monitor reported.

The Britain-based Syrian Observatory for Human Rights said the raids were carried out by U.S.-led coalition warplanes.

A spokesman for the U.S.-led coalition fighting Islamic State told Reuters that its forces had conducted strikes near al-Mayadin on May 25 and 26 and were assessing the results.

The Observatory said more than 40 children were among those killed in the strikes, which leveled al-Mayadin's municipality building.

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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

Many of the families had fled from Raqqa, Islamic State's stronghold to the northwest, which U.S.-backed Syrian Kurdish and Arab fighters are pushing towards in an offensive against the jihadists, the Observatory said.

Residents saw reconnaissance aircraft and warplanes circling the city at 7:25 p.m. (1625 GMT) before they fired missiles which struck two buildings, one of which was a four-story block housing Syrian and Moroccan families of Islamic State fighters.

More strikes took place after midnight.

Islamic State is losing ground in both Syria and Iraq under assault from an array of sometimes rival forces in both countries. Many of its fighters who have retreated from other fronts are massing in Syria's Euphrates basin area.

The U.S.-led coalition says it is careful to avoid civilian casualties in air strikes and investigates any that are reported to have taken place. (Reporting by Angus McDowall and John Davison; Editing by Alison Williams)

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