BEIRUT — A Sunni extremist group took over opposition-held areas of a provincial capital near the border with Iraq on Monday after expelling rival fighters from an al-Qaida-linked group, activists said.

The march by militants of the Islamic State group on the city of Deir el-Zour in Syria's oil-rich province along the border with Iraq brings the extremist Sunni group closer to a showdown with President Bashar Assad's forces. The group recently captured cities and towns in northern Iraq and merged them with much of the territory under its control in eastern Syria.

The Britain-based Syrian Observatory for Human rights said militants of the IS group took over opposition areas in Deir el-Zour on Monday. Fighters of the rival al-Qaida-linked Nusra Front group withdrew from the area after clashes, the Observatory said.

The new developments effectively expand and consolidate areas held by fighters from the Islamic State group in territory straddling the border between the two conflict-ridden countries of Syria and Iraq.

The group, which now controls large parts of northern Syria, is almost in full control of Deir el-Zour in the east, stretching from the Syrian border town of Boukamal to the provincial capital to the northwest. Assad's forces still control half of Deir el-Zour city, and no fighting between his troops and the extremist group was reported.

Led by an ambitious Iraqi militant known as Abu Bakr al-Baghdadi, the Islamic State group unilaterally declared the establishment of an Islamic state, or caliphate, in the lands it has seized in Syria and Iraq. It proclaimed al-Baghdadi the head of its new self-styled state and demanded that all Muslims pledge allegiance to him.

Most significant Syrian rebel groups that have been fighting to overthrow Assad have rejected al-Baghdadi's declaration. The rebel groups, including the Nusra Front, have fought the extremist group since the beginning of the year. Nearly 7,000 people, mostly fighters, have died in the infighting, and tens of thousands of civilians have been forced from their homes.

However, Nusra Front appears to be losing the war within a war in Syria as fighters allied with powerful tribes in eastern Syria defect to al-Baghdadi's group.

The Syrian conflict began in March 2011 as largely peaceful protests against Assad's rule. It deteriorated into an armed revolt after some opposition supporters took up arms to fight a brutal government crackdown on dissent. It became a civil war in which Islamic extremists, including foreign fighters and Syrian rebels, who have taken up hard-line al-Qaida-style ideologies, have played an increasingly prominent role among the fighters, dampening the West's support for the rebellion to overthrow Assad.