BEIRUT (AP) — Syrian activists say heavy clashes are underway between several rebel factions and an al-Qaida breakaway group over control of a border crossing with Iraq.

The Britain-based Syrian Observatory for Human Rights says Monday's fighting between rebel groups and rivals in the Islamic State of Iraq and the Levant is concentrated in the town of Boukamal on the border between Syria and Iraq.

The jihadist group, which on Sunday declared the establishment of an Islamic caliphate, controls much of northeastern Syria. In Iraq, it has recently captured cities and towns as well as border crossings, effectively erasing the frontier.

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.