Free Syrian Army fighters hold their weapons in northern Hama countryside Sunday.

BEIRUT: An offensive by a breakaway Al-Qaeda 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.

The Islamic State of Iraq and the Levant’s (ISIL) 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. 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-Qaeda-affiliated Nusra Front and other Islamic rebel groups.

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.

The Britain-based Syrian Observatory for Human Rights said the fighting in Deir el-Zour has killed 634 people since April 30. 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.

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.

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.

But the Islamic State has since consolidated its hold on Raqqa and the surrounding province. Then in the first week of May, its fighters pushed onward to the neighboring province of Deir el-Zour, blasting their way through towns along the Euphrates River and closing in on the provincial capital, the city of Deir el-Zour.

Also Tuesday, the government began releasing people from at least two prisons under a “general amnesty” offered by Assad following his re-election last week.

The attorney general for the countryside of Damascus, Ziad Al-Hulaibi, said the first batch of prisoners would be released Tuesday.

A police official in Damascus said an unknown number of prisoners were released Tuesday from the Adra prison northeast of the capital. The official spoke on condition of anonymity because he was not authorized to brief journalists.

Observatory director Rami Abdurrahman confirmed that some people were freed in Adra as well as in Aleppo, but said “we don’t know exactly how many have been released up till now.”

Syria’s state news agency has not said whether the amnesty would apply to the tens of thousands of anti-government activists, protesters, opposition supporters and their relatives that international rights groups say are imprisoned in the country. It does, however, cover foreign fighters, who will not be prosecuted if they surrender, according to SANA.

Fighters from the al-Qaeda breakaway group Islamic State of Iraq and the Levant (ISIL) march in Raqqa, Syria, in this file photo. An activist group said Tuesday that more than 630 people had been killed as of the end April in infighting between the ISIL and other anti-Assad groups.