The Britain-based Syrian Observatory for Human Rights said Boukamal fell to the militants early today following days of battles between the group and other factions led by the Nusra Front, al-Qaida's Syrian affiliate.
Activists in the area could not immediately be reached and calls to Boukamal and nearby areas were not going through. The Observatory, which has a network of activists around Syria, said the Islamic State brought in reinforcements from Iraq during the fighting.
The group says its Islamic state stretches from northern Syria to the Iraqi province of Diyala northeast of Baghdad, and has called on all Muslims worldwide to pledge allegiance to it.
The Observatory said the Islamic State released more than 100 detainees it was holding in the northern Syrian town of Al-Bab after the group's leader, Abu Bakr al-Baghdadi, issued an amnesty on the occasion of establishing the self-styled caliphate.
Last week, beleaguered Nusra Front fighters defected and joined the Islamic State in Boukamal -effectively handing over the town to the powerful group, which controls the Iraqi side of the crossing.
Up to 7,000 people, the majority of them fighters, have been killed in the rebel-on-rebel violence across the opposition-held territory in northern and eastern Syria since January, according to the Observatory's tally, which is compiled by its activists on the ground.
