An offensive by a breakaway al-Qaida group in eastern Syria against Islamic rebel factions has killed more than 630 people and uprooted at least 130,000 since the end of April, an activist group said Tuesday, according to Reuters.

The Islamic State of Iraq and the Levant’s campaign in the oil-rich Deir el-Zour province appears designed to link the large amount of territory under its control in northern Syria with the ground it also holds across the frontier in neighboring Iraq, Reuters reported.

The group, which is largely composed of foreign jihadists, has made significant headway over the past six weeks, seizing towns and villages in heavy fighting against the al-Qaida-affiliated Nusra Front and other Islamic rebel groups, according to Reuters.

The Islamic State has also made major gains in Iraq, where on Tuesday its fighters seized parts of Mosul, the country’s second-largest city, scattering security forces and capturing the provincial government headquarters and security bases, Reuters reported.

The Britain-based Syrian Observatory for Human Rights said the fighting in Deir el-Zour has killed 634 people since April 30, according to Reuters. It said the tally includes 39 civilians, 354 rebel fighters, including many from the Nusra Front, and 241 gunmen from the Islamic State.

The offensive in Deir el-Zour is part of a broader conflict between the Islamic State and Syrian rebels that has raged across opposition-held northern Syria since early January and killed more than 6,000 people, Reuters reported.

This war-within-a-war has been a massive drain on resources and manpower on the opposition in Syria, undermining its fight against President Bashar Assad in the wider civil war, according to Reuters.

Once spread across much of northern Syria, the Islamic State withdrew many of its far-flung fighters to its stronghold in the northern city of Raqqa earlier this year after other rebel factions, furious with the Islamic States’ efforts to impose its hard-line interpretation of Islam, launched an offensive against the group, Reuters reported.

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