A bomb attack targeting a weapons bazaar in eastern Syria near the Iraqi border killed 30 "terrorists" on Saturday, state television reported.
"A big explosion hit a terrorist arms market in Mayadeen, killing 30 terrorists and wounding dozens of others," the channel said.
The Syrian Observatory for Human Rights gave a different account, alleging a "bomb planted in the car of an arms dealer" caused a series of blasts, as nearby munitions exploded.
"At least eight civilians were killed and 21 others were wounded," said the Britain-based group, which distributed amateur video showing the aftermath.
Just 80 kilometres (50 miles) from the Iraqi border, Mayadeen is controlled by rebel groups, including Al-Qaeda's Syria affiliate the Al-Nusra Front, that have been fighting the jihadist Islamic State of Iraq and the Levant.
ISIL is the same cross-border group which has spearheaded a lightning offensive in neighbouring Iraq this week that has seen militants sweep down from second city Mosul towards Baghdad.
A rebel spokesman from Syria's Deir Ezzor province contested the state television version, and told AFP the blast was a car bomb planted by ISIL that killed at least 15 civilians in a street market.
Signup to our newsletter and follow us on Facebook and Twitter!
ISIL's fighters in Syria have been under attack by rival rebels since the start of the year.
They have been driven out of much of the northwest, but retain control of the city of Raqa up the Euphrates Valley from Deir Ezzor.
They have tried repeatedly to extend their area of control to the Iraqi border to unite their forces in the two countries.
In Deir Ezzor province, one of the main groups fighting ISIL has been Al-Nusra Front, which late Friday reportedly brought five Hummers and three other vehicles captured from the Iraqi army into Syria.
Violence between Syrian regime forces and rebels raged on, meanwhile, with new air strikes targeting Aleppo and Idlib provinces in the north and northwest, the Observatory said.
In Aleppo province, 13 people were killed in a barrel bomb attack on the rebel-held town of Anadan, while nine others including four women and a child were killed in air raids on Kafr Batikh in Idlib province.
Syria's war is estimated to have killed more than 162,000 people and forced nearly half the population to flee their homes.