Fighting between rival jihadist groups in eastern Syria killed 63 people on Monday, despite a call from al Qaeda`s chief for a ceasefire, the Syrian Observatory for Human Rights said.
The clashes between al Qaeda`s Syrian branch al Nusra Front and the Islamic State of Iraq and the Levant (ISIL) have prompted 60,000 people to flee towns in the oil-rich province.
The Observatory said 58 fighters on both sides had been killed in the clashes on Monday, along with five civilians.
The deaths bring to 150 the toll since the latest confrontation between the groups erupted last week in Deir Ezzor, bordering Iraq.
It comes after Islamist and moderate rebels eventually joined by al Nusra launched an offensive against ISIL earlier this year.
While al Nusra has been accepted as an ally by many rebels, ISIL has been criticised for its abuses of civilians and rival opposition groups.
The fighting with ISIL has killed around 4,000 people, according to the Britain-based Observatory.
The two jihadist groups have continued to clash despite a call from al Qaeda chief Ayman al-Zawahiri for an end to hostilities.
In an audio recording released on Friday, Zawahiri urged al Nusra to stop fighting ISIL and focus on battling the Syrian regime.
He also a repeated a call for ISIL to restrict its activities to Iraq, a plea that the group has rejected on several occasions.
In a response to Zawahiri`s call, Al-Nusra said it would stop fighting ISIL if the group ended its attacks.
"We will follow the orders of... Ayman al-Zawahiri... to stop any attack from our side against ISIL, while continuing to respond whenever they attack Muslims and all that is sacred to them," Al-Nusra said in a statement.
"As soon as ISIL announces the end of its attacks on Muslims, we will spontaneously stop firing," said the jihadist group, adding it had only fought ISIL in areas "where it was on the attack".