Internally displaced people who fled Raqqa ride in a vehicle in a camp near Ain Issa, Syria, on May 19. (Rodi Said/Reuters)

Airstrikes on an Islamic State-held town in eastern Syria killed dozens of civilians, monitoring groups said ­Friday, notably in an alleged raid on a building that housed family members of the extremist fighters.

The strikes on Mayadin, which took place Thursday evening and early Friday, were carried out by aircraft that locals identified as being from the U.S.-led coalition fighting the Islamic State, according to the Britain-based ­Syrian Observatory for Human Rights.

Capt. Jeff Davis, a Pentagon spokesman, said coalition forces conducted airstrikes in the area in Deir al-Zour province on Thursday and Friday.

“We are still assessing the results of those strikes,” Davis said. “We take all allegations of civilian casualties seriously.”

If confirmed as coalition airstrikes, the bombardment in Mayadin would mark the latest in a string of U.S.-led attacks that have killed perhaps hundreds of civilians in Iraq and Syria in recent months.

The Pentagon on Thursday released an unclassified summary of its investigation into a deadly March raid on a residential building in the Iraqi city of Mosul, where Islamic State fighters are holed up and battling Iraqi forces. The U.S. Central Command acknowledged that a U.S. strike killed more than 100 civilians there, but it blamed the high death toll on secondary explosions from Islamic State weapons stored in the area.

The United States has carried out more than 8,500 strikes in Syria since 2014, according to the Pentagon. An additional 8,700 U.S. strikes have been conducted in Iraq. Islamic State fighters are based in parts of the two countries, where they declared a “caliphate” in 2014.

Since President Trump took office in January, “we are now seeing the emergence of clear trends,” said Chris Woods, director of Airwars, a group that tracks coalition strikes in Iraq and Syria. “We are seeing high civilian casualties where six months ago we would not. This is the clearest evidence yet that protections for civilians on the battlefield appear to have been scaled back.”

In a statement Friday, U.N. human rights chief Zeid Ra’ad al-Hussein urged all air forces operating in Syria “to take much greater care to distinguish between legitimate military targets and civilians.”

“The same civilians who are suffering indiscriminate shelling and summary executions by [the Islamic State] are also falling victim to the escalating airstrikes,” he said. “The rising toll of civilian deaths and injuries already caused by airstrikes in Deir al-Zour and Raqqa suggests that insufficient precautions may have been taken in the attacks.”

Civilian casualties have risen as fighting has intensified ahead of a U.S.-backed assault on the Islamic State’s de facto capital in Raqqa in northern Syria. There, the Syrian Democratic Forces, led by ethnic Kurdish commanders, are pushing toward the city with the help of U.S. advisers.

Mayadin, in the oil-rich eastern province of Deir al-Zour, is roughly 100 miles from Raqqa and lies along Syria’s fertile Euphrates River basin. Most of that area is under the control of the Islamic State. The group — also known as ISIS and, in Arabic, Daesh — captured Mayadin from more-moderate Syrian rebel forces in 2014.

According to the Observatory for Human Rights, which relies on a network of local contacts to track the Syrian civil war, the first strikes took place at 7:25 p.m. Thursday. Two reconnaissance aircraft and two warplanes flew overhead before the latter fired four missiles at the town. Two reportedly struck the Dahmoush building, where displaced family members of Islamic State fighters from elsewhere in Syria were staying. The monitoring group said 42 children were among the dead, but that figure could not be corroborated with other activists or sources in the area.

Then, early Friday, the warplanes returned and struck the municipal building, the local power company and the blood bank. At least 10 Islamic State fighters were killed, the Observatory said.

A separate local monitoring group said that it had dispatched a fact-finder to assess the aftermath of the strikes in Mayadin but that Islamic State fighters had blocked the entrances to the town. The activist group said the strikes took place in the Souk al-Mokhales and Saraya areas of Mayadin. Another strike hit a market, according to local media outlets operated by rebel-linked activists, and a separate missile reportedly landed between two hospitals.

The U.S. Central Command on Friday listed five strikes in Deir al-Zour, which it said destroyed “a command and control node, and an ISIS headquarters.”

Some analysts have said the Islamic State has moved family members and some assets to Mayadin as the U.S. ramps up pressure on the group in Raqqa.

Also Friday, the Central Command said that coalition strikes in Mayadin on April 27 and May 11 killed two Islamic State commanders involved in planning the group’s attacks abroad.

The commanders were identified as Mustafa Gunes, a Turkish national, and Abu Asim al-Jazaeri, a French Algerian fighter. In April, the U.S. military said Special Operations commandos killed a senior Islamic State commander, Abdurakhmon Uzbeki, in a raid in Mayadin.

The aerial bombardments have been “unusually intense in the last two weeks” around Mayadin and other areas of Deir al-Zour, said Omar Abu Leila, the Germany-based editor of Deir Ezzor 24, a local independent news outlet.

Mayadin is home to tens of thousands of displaced Syrians, including civilians, according to Abu Leila and international rights groups.

Both the locals and the displaced “are all deathly afraid now” of the airstrikes and rising violence in the area, he said. “The airstrikes come so suddenly . . . and people even get kicked out of places to make room for the families of Islamic State fighters.”

But “it’s all the same now,” he said. “The Russians, the international coalition and Daesh. The situation is extremely tense.”

Heba Habib in Stockholm and Missy Ryan in Washington contributed to this report.