Islamic State in Iraq and the Levant (ISIL) fighters launched an offensive on Monday, taking advantage of a sandstorm that sharply reduced visibility in eastern Syria and capturing areas from government forces near the city of Deir al-Zour, said the Syrian Observatory for Human Rights, an independent monitoring group.

The organization, based in Britain, said the combatants captured areas just north of the city, including an army base known as the Saiqa camp and parts of arms depots in the village of Ayash. Opposition activist Omar Abu Leila reported that ISIL fighters captured the entire village, which is just north of another town that ISIL captured over the weekend.

The Observatory and Abu Leila, who is from Deir al-Zour but currently lives in Europe, said the sandstorm reduced visibility in Deir al-Zour, giving advantage to ISIL fighters who pressed their offensive without fear of being hit from the air by Syria's air force, grounded by the bad weather.

Over the weekend, ISIL captured several areas in Deir al-Zour province, killing scores of troops and civilians and capturing hundreds in this province bordering Iraq, according to the Observatory and state media.

ISIL appears to be trying to reverse a string of defeats in Syria and Iraq over the past months. It controls most of Deir al-Zour province and much of the capital with the same name, while the government controls several districts in the northern part of the city and the adjacent military airport. Most of the casualties in the latest fighting occurred in the area of Baghaliyeh near the city.

If ISIL captures the whole city of Deir al-Zour, it will be a major boost for the group as they will control two provincial capitals in Syria. ISIL already controls the northern Syrian city of Raqqa — the group's de facto capital — in the province by the same name.

"Daesh is trying to capture Deir al-Zour to achieve a victory after its recent failures in Syria and Iraq," said the Observatory's chief Rami Abdurrahman, referring to ISIL by its Arabic acronym.

Syria's government said Sunday that ISIL killed 300 people in an "appalling massacre" over the weekend in the city of Deir al-Zour. The Syrian Foreign Ministry said Baghaliyeh was the scene of a "bloody massacre committed by the Daesh terrorist organization."

Abdurrahman said areas under the control of the government in Deir al-Zour city are home to some 200,000 people who have been under siege for months. He added that Monday's fighting left dozens of troops dead or wounded.

Abu Leila, who heads the Deir Ezzor 24 news network, said clashes on the northern edge of the city focused around the Heart Hospital. The Observatory and Deir Ezzor 24 reported that among the dead Monday was a senior army officer, Brig. Gen. Samer Amin Ali.

ISIL, which controls large parts of Syria and Iraq where it declared an Islamic caliphate in June 2014, suffered several defeats recently in both countries, including the loss of the Iraqi city of Ramadi and parts of northern and northeastern Syria over the past months.

ISIL has tried to capture the government-held neighborhoods of Deir al-Zour and the city's suburbs over the past months without much success.

The latest offensive began at dawn Saturday when ISIL fighters crossed the Euphrates River in small boats and stormed government positions on the west bank, according to Abu Leila. The attack was preceded by intense shelling of troop positions by ISIL fighters.

Abdurrahman confirmed the sequence of events and added that on Saturday alone, 42 fighters were killed, including two who detonated their vehicles and 10 who blew up their explosive vests.

Abu Leila, said a feared ISIL unit known as Karar Army has been instrumental in the group’s surge in the province.

The Associated Press