BEIRUT -- Militants with the Islamic State of Iraq and Syria (ISIS) launched a fresh offensive Monday, taking advantage of a sandstorm that sharply reduced visibility in eastern Syria and capturing new areas from government forces near the city of Deir el-Zour, opposition activists said.

The Britain-based Syrian Observatory for Human Rights said the militants 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 ISIS fighters had in fact captured the entire Ayash village, which is just north of another town that ISIS captured over the weekend.

The Observatory and Abu Leila, who is from Deir el-Zour but currently lives in Europe, said the sandstorm reduced visibility in Deir el-Zour, giving advantage to ISIS 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, ISIS captured several areas in Deir el-Zour province, killing scores of troops and civilians and capturing hundreds in this province bordering Iraq, according to activists and state media.

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The extremist group appears to be trying to reverse a string of defeats in Syria and Iraq over the past months. ISIS controls most of Deir el-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.

Syria's government said Sunday that ISIS killed 300 people in an "appalling massacre" over the weekend in the city of Deir el-Zour. The Syrian Foreign Ministry said Baghaliyeh was the scene of a "bloody massacre committed by the Daesh terrorist organization," referring to ISIS under its Arabic acronym.

The Observatory's chief Rami Abdurrahman said areas under the control of the government in Deir el-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.

ISIS, 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.