The Syrian Democratic Forces (SDF) suffered record fatalities in an "Islamic State of Iraq and Syria" (ISIS) assault, as the extremists kept up a fierce defence of their last Syrian holdout, AFP reported Monday (November 26th).

More than 200 people have been killed since around 500 ISIS fighters burst out of the fog shrouding eastern Syria near the border with Iraq to launch their deadly assault on Friday, the Syrian Observatory for Human Rights said.

According to the Observatory, 92 of the dead were SDF fighters. At least 61 extremists and 51 civilians, mostly their relatives, also died in the violence.

International coalition airstrikes helped the SDF recover positions it had briefly lost.

Local tribal fighters opposed to ISIS also helped the SDF take back the positions they had lost on Friday, Observatory chief Rami Abdel Rahman said.

The extremists used suicide bombers, suicide commandos and sleeper cells in the countryside around their bastion of Hajin to inflict maximum damage, the Observatory said.

Most of their recent forays and their deadliest attacks have come as a result of weather conditions hampering the coalition's ability to launch airstrikes.

The coalition said in a statement on Saturday that its strikes had been limited due to the weather.

"It is the largest number of SDF fighters killed (by ISIS) in a single battle since it was founded" in 2015, Observatory chief Rami Abdel Rahman said.

The latest deaths brought to 452 the number of SDF fighters killed since the start of their offensive on the Hajin pocket on September 10th, he said.

The Observatory said the death toll continued to soar on Sunday as many bodies were discovered.