U.S-led coalition airstrikes on the last pocket of land held by ISIS militants in Syria killed at least 40 people, mostly women and children, a war monitor and Syrian state media reported.

The coalition confirmed strikes in the area but said no "civilian casualties are associated" with them, reiterating that it takes measures to avoid non-combatant casualties.

"We have witnessed [ISIS] using places of worship and hospitals as command centres against the laws of war, and innocent civilians as human shields," Col. Sean Ryan, a spokesperson for the coalition, said in an email to The Associated Press.

The remote area near the border with Iraq is difficult to access and it was not possible to independently verify the reports.

However, Rami Abdurrahman, head of the Britain-based Syrian Observatory for Human Rights, said the early Saturday airstrikes hit homes in Abu al-Hassan village, near the town of Hajin, which lies along the border with Iraq.

Death toll uncertain

Abdurrahman said the airstrikes killed at least 43 people, including 17 children and 12 women. He said it was not immediately clear if the men killed in the strikes were militants.

Syrian state news agency SANA also reported the strikes, saying 40 were killed in the remote area of Buqaan, another village next to Abu al-Hassan, in Deir el-Zour province.

The ISIS-linked Aamaq news agency also reported 40 killed, quoting a medical official in the Hajin area.

Activist Omar Abou Leila, who monitors the war in Deir el-Zour from Europe, also confirmed the strikes, but said it was difficult to verify the death toll. Abou Leila said ISIS militants are preventing civilians from leaving the area, resulting in the high casualty toll among them.

Months-long offensive

The U.S.-led coalition and its local partners on the ground, the Kurdish-led Syrian Democratic Forces, launched the campaign against the last ISIS-held pocket in early September.

The observatory has recorded at least 191 civilians killed since Sept. 10, including 65 children and 45 women. They are mostly Iraqis and believed to include family members of ISIS militants, the organization added.

Speaking to The Associated Press in Iraq Saturday, the coalition's deputy commander Maj. Gen. Christopher Ghika described the fight against ISIS as "difficult."

"We never thought or said this fight would be easy. These are some of the most determined fighters and they've had a lot of time to prepare their defensive positions, so this isn't an easy fight, and our Syrian democratic force partners with coalition support are taking the fight every day to the enemy," Ghika said.