BEIRUT: Two senior ISIS leaders were killed in an air raid in northeastern Syria Monday, the Syrian Observatory for Human Rights.

The Observatory identified the two leaders as an Iraqi, Abu Osama al-Iraqi, and a Syrian named Amer al-Rafdan.

The Observatory said the airstrike was believed to have been carried out by the U.S.-led coalition that is targeting ISIS in Syria and Iraq, while at least one anti-regime activist group in Hassakeh, where the strikes took place, said the coalition was responsible.

ISIS is fighting in Hassakeh province against both pro-government forces and Syrian Kurdish fighters who have received air support from the U.S.-led coalition.

U.S. President Barack Obama said last week the coalition was intensifying its campaign in northern Syria. The Kurdish YPG militia have made a series of advances against ISIS in the north, including the capture of the town of Tal Abyad at the Turkish border.

The Observatory said Iraqi had the title of “governor” of the province ISIS had declared in northeastern Syria. Rafdan had previously served the ISIS governor of Deir al-Zor province, he said.

The news came as Iraqi troops and Shiite militia forces attacked ISIS fighters on several fronts in Anbar province, at the start of what is likely to be a long and fiercely contested offensive.

A spokesman for the joint operations command said the campaign, which began at dawn, brought together the army, mainly Shiite Popular Mobilization militias, special forces, police and local Sunni tribal fighters.

“At 5 o’clock this morning operations to liberate Anbar were launched,” the spokesman said.

Military sources in Anbar said they met heavy resistance from the insurgents, who deployed five suicide car bombs and fired rockets to repel their advance on Fallujah, about 50 km west of Baghdad.

Iraqi forces also pushed toward the provincial capital Ramadi from the west and the south, police sources in the province said. ISIS supporters said those advances were repelled by the militants.

ISIS’ capture of Ramadi two months ago marked the biggest defeat for the Baghdad government since the militants swept through the north of the country last June and declared a caliphate in parts of Syria and Iraq straddling the border.

Prime Minister Haider al-Abadi, angered at the army’s withdrawal from Ramadi in mid-May, initially promised a swift counterattack. Although Ramadi remains the strategic target, military sources and Shiite militia leaders have said the initial focus will be on Fallujah, which was the first city in Iraq to fall under militant control 18 months ago.

A U.S.-led coalition of Western and Arab air forces has been bombing ISIS positions across Iraq, supporting Baghdad’s ground forces and its poorly equipped air force.

Iraq has sought to redress its own lack of air power by purchasing F-16 fighter jets from the United States, but deliveries of the 36 aircraft were delayed because of security concerns after the ISIS offensive last summer.

The first batch of four F-16s Monday landed at Balad air base north of Baghdad, an Iraqi air force officer said. It was not clear whether the planes would take an immediate role in the operations in Anbar.

Hadi al-Ameri, commander of the largest Shiite force, the Badr Organization, told Iraqi television Sunday that he expected the main assault on Fallujah to take place after the Eid holiday which starts later this week.

Residents in Fallujah and Ramadi reported heavy bombardment of both cities early Monday.

Abadi initially sought to keep Shiite militias on the sidelines in Anbar for fear of inflaming sectarian sensitivities, but the fall of Ramadi in May undermined his position.

In Baghdad Sunday at least 35 people were killed in a series of car bombs and suicide attacks in mainly Shiite districts. A statement issued Monday in the name of ISIS claimed responsibility for some of the blasts.