(CNN) -- Rival Islamist factions battled each other for a second straight day in eastern Syria on Friday -- a fight that so far has killed 68 militants, a Britain-based Syrian opposition group said.
The clashes in a desert of Syria's Deir Ezzor province near the Iraqi border are part of a war within a war -- Islamist groups fighting each other for supremacy even as they oppose, in Syria's larger civil war, the forces of Syrian President Bashar al-Assad.
The latest fighting between the al Qaeda splinter group Islamic State in Iraq and Syria (ISIS) and other Islamist battalions -- including the al Qaeda-linked al-Nusra Front -- began in that area Thursday, the Syrian Observatory for Human Rights said.
Fighting continued Friday around Bukamal city and the town of Kabajeb, the observatory said.
Of the 68 killed, 53 were fighters from al-Nusra Front and their allies, according to the observatory.
Last year, ISIS launched a successful offensive to wrest control of large swaths of northern Syria and push rebels out of key population centers. The aggression helped make ISIS the target of al-Nusra Front and other rebel factions.
In February, al-Nusra Front demanded that ISIS leave Syria. Intra-rebel fighting has killed more than 3,300 people since the start of this year, according to the observatory.
Well over 100,000 people have been killed and millions displaced since the Syrian civil war began in 2011, the United Nations says.
CNN's Mohammed Tawfeeq contributed to this report.