BEIRUT --- An airstrike at a mosque in a rebel-held area of northern Syria on Thursday killed at least 35 people, first responders and activists said.

The Syrian Civil Defense, volunteer paramedics known as the White Helmets, said first responders were racing to the scene after the airstrike in the Jeeneh area, near the rebel-held province of Idlib. It said at least 35 people were killed.

The Britain-based Syrian Observatory for Human Rights said at least 46 people were killed, mainly civilians.

Get Breaking News Delivered to Your Inbox

Bilal Abdul Kareem, an American journalist with close links to militant Islam groups in Syria, is reporting that he is at the mosque. He put the death toll at 56.

The Reuters news agency reports that the powerful Syrian rebel group Ahrar al-Sham said Friday the U.S.-led coalition fighting the Islamic State of Iraq and Syria (ISIS) was behind the mosque attack.

U.S. officials say the strike killed dozens of militants and they’re investigating reports that civilians were killed. U.S. Army Maj. Josh Jacques, a U.S. Central Command spokesman, said the U.S. did not target or strike a mosque.

“We targeted an al Qaeda gathering across the street from a mosque. The mosque does not appear to be damaged following the strike,” said U.S. Army Maj. Josh Jacques, a U.S. Central Command spokesman. “We are aware of the reports of civilian casualties and we are looking into it.”

The U.S. Central Command based in Tampa, Florida, said separately that U.S. forces killed “several terrorists” in a strike at an al Qaeda meeting place in Idlib. It said in a statement that Idlib has been a significant safe haven for al Qaeda in recent years. The U.S. has struck dozens of locations in northwestern Syria in the past, targeting al Qaeda-linked militants.

Col. John J. Thomas, a spokesman for the U.S. Central Command, there was a mosque across the street from the building the U.S. hit on March 16. He said post-strike photos show two craters and half the target building destroyed, with the small mosque across the street still standing.

However other reports indicate the building may have been a newer annex to the old mosque.

Later Friday, DOD spokesman Eric Pahon told CBS News’ David Martin that intelligence indicated that al Qaeda leaders used this partially-constructed community meeting hall as a gathering place, and as a place to educate and indoctrinate al-Qaeda fighters

“Initial assessments based upon post-strike analysis do not indicate civilian casualties. Again, this was a meeting place for al-Qaida leaders and fighters. We take all allegations of civilian casualties seriously and will investigate any credible allegations we receive,” Pahon said.

The Pentagon released this image of a March 16 airstrike in al-Jinah, Syria. In the center is the building struck in the airstrake, which the Navy says was the site of an al Qaeda senior leader meeting. The Navy says they deliberately did not target the mosque at the left edge of the photo, and the area was extensively surveilled prior to the strike. Pentagon

Jeeneh is in the western Aleppo countryside which, along with Idlib, is home to hundreds of thousands of Syrians displaced by fighting in other areas.

Russian and Syrian aircraft are known to operate in the opposition-held region.

The airstrike on the mosque came a day after suicide attacks in the capital, Damascus, killed at least 30 people on the sixth anniversary of the start of the Syrian conflict.

The civil war has killed about 400,000 people, wounded more than a million and displaced half the country’s population.