ISIS militants killed 11 Iraqi soldiers in an overnight raid on army headquarters in the eastern province of Diyala, the state news agency said on Friday.
The attack was one of the deadliest carried out by the extremist group against Iraqi security forces in recent months.
The army's Diyala Operations Command sent reinforcements to the headquarters in Hawi Al Azim district and opened an investigation.
“The attack was carried out against members of the First Division in the Al Azim area that lies on the borders between Diyala and Salaheddin governorate,” Diyala Governor Muthanna Al Tamimi said in a statement.
Mr Al Tamimi said the army were unprepared for the attack.
“The main reason [for this attack] is negligence on the part of soldiers because the base is fortified,” he said.
“There is a thermal camera, night-vision goggles and a concrete watchtower.”
Frequent attacks along the border between Diyala and Salaheddin have been blamed on a lack of security co-ordination between the two provinces.
Iraq declared victory over ISIS in late 2017 after reclaiming northern and western regions seized by the extremist group in 2014.
But remnants of the group have been carrying out a low-level ISIS insurgency, particularly in the north, that is disrupting efforts to restore stability to Iraq after years of warfare and sectarian unrest.
The country currently faces a political vacuum as parliamentary groups try to agree on the formation of a new government following a general election in October.
Iraqi President Barham Salih said the latest attack in Diyala targeted the country's security and that attempts to revive terrorism cannot be taken lightly.
“Our duty is to form a capable government that protects national security and serves the people,” Mr Salih said on Twitter.
Prime Minister Mustafa Al Kadhimi said the crime would "not pass without decisive punishment."
"Our forces will have a resounding response against the murderous terrorists. Our heroic armed forces have a duty to prevent the recurrence of these violations and to pursue terrorists everywhere, for the sake of Iraq and the Iraqis," he wrote on Twitter.
Last October, ISIS militants raided a predominantly Shiite village in Diyala province, killing 11 civilians and wounding several others.
Officials said the attack occurred after the militants kidnapped villagers and their demands for ransom were not met.
The previous month, ISIS fighters killed 10 policemen and injured four in an attack on a guard post near the northern city of Kirkuk.
Police sources said the attackers clashed for two hours with police stationed at a village in the town of Rashad, 30 kilometres south-west of Kirkuk.
In July, ISIS claimed the bombing of a market in Sadr city, a Shiite suburb of the capital Baghdad, that killed dozens of people.
A UN report last year estimated that about 10,000 ISIS fighters remain active in Iraq and across the border in Syria.