BEIRUT (Reuters) - Unidentified gunmen killed at least 12 Islamic State militants in the eastern Syrian town of al-Mayadin in an overnight attack in an area near the Iraqi border controlled by ISIS, a group monitoring the conflict reported.

Gunmen on at least two motorbikes first opened fire on an Islamic State patrol before attacking Islamic State militants guarding a nearby courthouse, the Syrian Observatory for Human Rights reported.

A second group of gunmen meanwhile attacked an Islamic State checkpoint in the same town, killing and injuring an unknown number of the Islamic State militants, the Observatory reported.

Al-Mayadin is in the province of Deir al-Zor which is mostly controlled by Islamic State and is located about 60 km (40 miles) from the border with Iraq.

(Writing by Tom Perry Editing by Jeremy Gaunt)