Fighters from al Qaeda's Nusra Front withdrew from two towns in eastern Syria on Thursday, leaving most of the border province of Deir al-Zor under the control of advancing forces of the Islamic State, the Syrian Observatory for Human Rights said.
It said the Nusra Front pulled out of Mayadin and Shuhail, the group's regional stronghold, while local tribal fighters had pledged allegiance to the Islamic State, which has also swept through Sunni Muslim provinces in Iraq.
The Observatory, a British-based monitoring group, said the Islamic State, previously called the Islamic State in Iraq and the Levant (ISIL), now controls an area of Syria five times the size of neighbouring Lebanon.
In Deir al-Zor province only the regional capital and airport - still held by President Bashar al-Assad's forces - and a few villages remain outside the Islamic State's control, it said.
Earlier this week the Islamic State seized the town of Albu Kamal on the Iraqi frontier from the Nusra Front, securing both sides of the border crossing.
"For the first time the Islamic State has taken control over territory extending from the strategic town of Albu Kamal on the Iraqi-Syrian border to the north-eastern edge of Aleppo," it said. Albu Kamal is around 400 km (250 miles) from Aleppo.
Islamic State fighters seized control of several Iraqi towns in a lightning offensive in June. The group says it is restoring an Islamic caliphate on the cross-border territory it now controls and has called on Muslims to join it in jihad.