In some of the fiercest clashes in Syria in recent months, around 50 US-backed fighters have been killed in battles with ISIS in Deir Ezzor province since Friday.
“ISIS leveraged sandstorms, and a fog to launch a large counterattack against our forces and infiltrate areas that we had liberated recently,” Mustafa Bali, a spokesman for the Syrian Democratic Forces told The National.
The latest counterattack is not the first time SDF fighters have been forced to retreat since the start of a nearly three-month-long offensive against ISIS, which has so far failed to achieve major gains.
This suggests that militants still command sufficient defensive and offensive capabilities that allow them to hold the little ground they still control in parts of eastern Syria.
Omar Abu Leila, the head of the activist-run DeirEzzor24 news website, said up to 50 SDF fighters were killed in ISIS counter-attacks in the villages of Al Bahra and Gharanij across the past 72 hours. At least 20 others were taken hostage by the militant group, he told The National.
He also said militants killed at least two civilians, while 30 others are missing.
The UK-based Syrian Observatory for Human Rights put the number of SDF fighters killed at 92 since Friday. The monitoring group claims about 61 militants and 51 civilians, mostly their relatives, also died in the violence which saw US-led coalition air strikes targetting militant positions in the Hajin pocket
The SOHR described it as one of the "biggest and fiercest" battles in recent months, adding that it has caused one of the highest death tolls compared to recent fighting.
The SDF spokesman refused to disclose an exact casualty count among SDF fighters but said that he believes the number provided by the SOHR is “exaggerated.”
When asked to confirm whether 20 SDF fighters have been taken captive, Mr Bali said that some of his fighters “may have been captured,” but he refused to disclose an exact figure.
He said that his forces have killed scores of ISIS militants and have detained a number of fighters following fierce battles.
The SDF, a Kurdish-led alliance, has been battling to oust ISIS from the area near the Iraqi border since September.
The Observatory says 452 SDF fighters and about 739 ISIS militants have been killed since a US-backed campaign was launched nearly three months ago.
Mr Omar and the SOHR said a convoy of militants drove under the cover of heavy fog and sandstorms on Friday, penetrating the two areas near the Al Tanak oilfield, which is manned by SDF and coalition forces.
Mr Omar and the SOHR said coalition warplanes were flying overhead at the time of the attack but failed to intercept the militant convoy, despite receiving warnings from nearby residents, including groups fighting with the SDF.
The National reached out the US coalition but it was not available for comment.
ISIS fighters have since withdrawn from the area.
Eastern Syria's Hajin area is the last significant remnant of the so-called caliphate ISIS proclaimed in 2014. The rest has been lost to offensives launched by different factions in both Syria and Iraq.
Outside Hajin, the group's operations are confined to sleeper cells and to hideouts in unpopulated desert and mountain areas.
Continued ISIS operations in Hajin have caused security concerns across the border in Iraq
Abu Kameel a member of the Iraqi Popular Mobilization Forces told The National that there were no clashes near the Iraqi border this week.
However, Qais al-Khazali, the leader of Iranian-backed Asaib Ahl al-Haq group which is part of the PMF, urged the government on Saturday to provide a more formal, long-term border protection role for the militias.
"Securing Iraq's borders with Syria is among the most important duties of the Popular Mobilization Forces right now," he said in an interview with Reuters at his office in the Shi'ite holy city of Najaf on Saturday.
"The Daesh (IS) threat against Iraq won't end as long as Syria is unstable. The PMF proved it is the military side most capable of dealing with Daesh ... maybe the armed forces can invest the PMF in duties that include border security," Mr Khazali said.