This March 2016 photo shows the Roman amphitheater in the ancient city of Palmyra in central Syria. (Joseph Eid/Agence France-Presse via Getty Images)

Islamic State militants have partially destroyed a Roman amphitheater that they are using for public executions in the ancient Syrian city of Palmyra, state officials said Friday.

The extremist group seized control of the area last month for the second time, reversing one of Russia’s largest military victories in Syria and raising fears that the militants would destroy what remained of one of the Middle East’s most famous Roman sites.

Once a major stop for ­cross-desert caravans, the 3rd century limestone and dolomite city was ruled by the legendary Queen Zenobia and stood as a testament to the area’s ­cosmopolitan history.

Irina Bokova, head of UNESCO, condemned the latest demolitions as a “war crime.”

“This new blow against cultural heritage, just a few hours after UNESCO received reports about mass executions in the theater, shows that cultural cleansing led by violent extremists is seeking to destroy both human lives and historical monuments,” she said.

The Tetrapylon in Palmyra, Syria, on March 31, 2016. (Joseph Eid/Agence France-Presse via Getty Images)

Islamic State militants have already destroyed two of its temples and the huge triumphal arch honoring Roman Emperor ­Septimius Severus.

“This is not just the destruction of ancient ruins, this is the destruction of civilization,” said Maamoun Abdulkarim, Syria’s antiquities minister.

Satellite imagery appears to show the destruction of large parts of the amphitheater’s facade and structures on the stage. The Tetrapylon, a towering ­structure marking the intersection of the city’s two main ­boulevards, also appears to have been destroyed.

Like many of the country’s ancient treasures, Palmyra’s ­ruins have been looted by government forces, damaged in fighting and airstrikes and shattered with dynamite during its occupation by the Islamic State — a group that has delighted in destroying archaeological sites in Syria and Iraq, labeling them as pagan.

Russian airstrikes facilitated the Syrian government’s ­recapture of Palmyra in March, ­marking Moscow’s most concrete ­victory at the time against the militants since it intervened to prop up President Bashar ­al-Assad’s then-crumbling ­fortunes.

But Islamic State forces met little resistance when they moved to retake the area in December.

According to Abdulkarim, ­reports of fresh damage to the ancient city trickled out in late December. The satellite imagery, provided 10 days ago to the Syrian government, confirmed them.

Khaled al-Homsi, an activist from the area, said Thursday that the extremist group had executed 12 prisoners, some on the ­amphitheater stage. He said that most of the dead were believed to be Syrian soldiers or ­government-allied militiamen, and that four were teachers or government ­officials.

Once one of Syria’s most popular tourist destinations, Palmyra was believed to be largely deserted by the time it fell to the Islamic State for a second time. Homsi said the area was mostly home to government forces and militiamen, with fewer than 100 civilians remaining.

Extremist groups have thrived in Syria during more than five years of war, and militants linked to the Islamic State and al-Qaeda now hold territory, or at least influence, across much of the country.

More than 100 suspected al-Qaeda militants were believed to have been killed in a U.S. operation targeting a training camp in a rural area of Idlib province, west of Aleppo, a U.S. defense official said Friday.

A team of B-52 aircraft, based in Qatar, and a number of drone aircraft conducted the operation Thursday, dropping a total of 14 munitions, the official said.

The camp was an important training site for what U.S. ­officials identify as “core al-Qaeda,” a group more directly linked to the international radical Islamist movement founded by Osama bin Laden than the group formerly known as Jabhat ­al-Nusra, a much larger organization that is identified as ­al-Qaeda’s Syrian branch but has been focused primarily on the fight against Assad.

According to the official, who spoke on the condition of anonymity to discuss an incident that has not yet been publicly announced, al-Qaeda militants began moving into western Syria in bulk in early 2016, operating alongside Nusra positions. The core al-Qaeda operation, U.S. officials say, differs in that it is more focused on planning ­attacks outside of Syria.

The group is believed to number in the low hundreds in Syria. The official said U.S. attacks have killed more than 150 of those militants since the beginning of the year.

Ryan reported from Washington.