At least 40 people, mostly women and children, have been killed by US-led airstrikes on the last pocket held by Islamic State militants in eastern Syria, according to a war monitor and Syrian state media.

Key points: The airstrikes reportedly hit homes in Abu al-Hassan village

The airstrikes reportedly hit homes in Abu al-Hassan village The Syrian Observatory for Human Rights has recorded at least 191 civilians deaths since September 10

The Syrian Observatory for Human Rights has recorded at least 191 civilians deaths since September 10 The coalition's Deputy Commander Major General Christopher Ghika described the fight against IS as "difficult"

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

Mr 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.

However the US-led coalition denied civilians were killed.

It said in a statement it conducted 19 strikes in the area in support of ground operations against IS.

It said all the targets it struck were "legitimate" and no civilians were present.

It said another 10 strikes in the area were not carried out by the coalition.

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

Activist Omar Abou Leila, who monitors the war from Deir el-Zour, also confirmed the strikes but said it was difficult to verify the death toll.

The activist said IS militants were preventing civilians from leaving the area, resulting in the high casualty toll among them.

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

The Observatory has recorded at least 191 civilians killed since September 10, including 65 children and 45 women.

They are mostly Iraqis and believed to include family members of IS militants, the Observatory added.

Speaking on Saturday, the coalition's Deputy Commander Major General Christopher Ghika described the fight against IS as "difficult".

"We never thought or said this fight would be easy," he said.

"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."

AP