Dusan Vranic, Associated Press
Syrian soldiers celebrate Bashar Assad's presidential re-election in Damascus, Syria, Wednesday June 4, 2014. Assad has been re-elected in a landslide, officials said Wednesday, capturing another seven-year term in the middle of a bloody 3-year-old uprising against his rule that has devastated the country.

BEIRUT — Fighting in eastern Syria between Islamic rebel brigades and an al-Qaida splinter group has killed more than 630 people and uprooted at least 130,000 since the end of April, an activist group said Tuesday.

The clashes in Deir el-Zour province, which borders Iraq, are part of broader rebel-on-rebel clashes that have raged across opposition-held northern Syria since early January. The violence pits rebel groups, including the al-Qaida-affiliated Nusra Front, against the Islamic State of Iraq and the Levant, which was disavowed by the terrorist network earlier this year because of its brutality.

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. Since January, more than 6,000 people have been killed in the infighting, according to activists.

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

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 since has consolidated its hold on Raqqa and the surrounding province. Then in early May, its fighters pushed onward to the neighboring province of Deir el-Zour, capturing villages and 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.

A police official in Damascus said an unknown number of prisoners were released Tuesday from the Adra prison northeast of the capital.

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.

Associated Press writer Albert Aji in Damascus, Syria, contributed to this report.