(Refiles to amend headline)
MOSUL, Iraq, May 11 (Reuters) - Militants in northern Iraq ambushed an army convoy and abducted at least 20 soldiers before shooting them in the head, army officials said on Sunday, the deadliest yet in a growing series of execution-style attacks in the country.
The ambush took place on Saturday near Ain al-Jahash in the northern province of Nineveh, one of the most unstable regions in Iraq and through which a long section of pipeline runs on its course from the Kirkuk oilfields to neighboring Turkey.
Security officials said militants disguised as soldiers and driving army vehicles, presumably seized in previous attacks, had ambushed the convoy and abducted the conscripts, killing them en masse hours later.
"The soldiers were taken by surprise and realised too late that the humvees were driven by terrorists rather than their colleagues," said one army officer from the same brigade as the slain conscripts.
No group immediately claimed responsibility for the killings, but insurgents including the Islamic State in Iraq and the Levant (ISIL) have a firm presence in Nineveh, which borders Syria.
"All the soldiers' bodies bore bullet wounds to the head... they were all executed and this is the hallmark of ISIL groups," said an army officer in Mosul. Two other officers also blamed ISIL.
The Kirkuk-Ceyhan pipeline has been idled for more than two months since insurgents bombed it in Ain al-Jahash. Technicians sent to fix the damage have been attacked and killed, putting repair work on hold.
This outage is the longest in decades and has held back oil export growth. (Reporting by Ziad al-Sanjary in Mosul; Writing by Ahmed Rasheed; Editing by Isabel Coles and Raissa Kasolowsky)