Militants in pick-up trucks ambushed Iraqi army outposts protecting a major oil export pipeline in the north of the country overnight, killing at least 16 soldiers by shooting them and slitting their throats, security and medical sources said.
No group claimed responsibility for the attack, but Sunni Islamist and other insurgents have been regaining momentum in a campaign to destabilise Iraq's Shi'ite-led government.
The attack took place near the Ain al-Jahash area, 30 km (20 miles) south of the city of Mosul in Nineveh province, through which a long section of pipeline stretches from Iraq's Kirkuk oilfield to neighbouring Turkey.
"Dozens of gunmen in pick-up trucks launched orchestrated attacks against army commando soldiers protecting an oil pipeline," said a security source, adding that an army humvee vehicle had gone missing during the attack. "The soldiers were taken by surprise and this is why we have such a high death toll."
The OPEC member's ambitious plans to ramp up its oil output have been held back by poor maintenance, technical problems and now deteriorating security.
More than 1000 people were killed in attacks across the country in January alone, building on a trend of intensifying violence that made last year the bloodiest since 2008, when sectarian warfare began to abate from its height.
"We have received 16 bodies of soldiers bearing bullets-wounds and with slit throats," said a doctor in Mosul hospital on condition of anonymity as he was not authorized to speak to media.
In a separate incident, an Iraqi daily newspaper stopped publishing after two bombs were planted in the entrance to its headquarters in Baghdad on Monday and after threats from an Iranian-backed Shi'ite militia.
Editors and reporters at Assabah AlJadeed said they had received death threats from the influential Asaib al-Haq militia in response to what it had described as an "insulting caricature" of Shi'ite Iran's Supreme Leader Ayatollah Ali Khamenei.
Some Iraqi Shi'ite groups that follow Khamenei, including Asaib al-Haq, are fighting in Syria, often alongside President Bashar al-Assad's troops and against mainly Sunni rebels.
Dozens of angry protesters carrying photographs of Asaib al-Haq leader Qais al-Khazali gathered in Baghdad's Firdos Square on Monday demanding the paper be shut down and whoever insulted revered Shi'ite figures, punished.
Firdos Square was the site of the famous statue of former dictator Saddam Hussein that was torn down after the US-led invasion of 2003, setting in motion the ascendancy of Iraq's Shi'ite majority.
The paper said on its website it had become a target of "irresponsible accusations" by extremist religious groups and said the photo of Khamenei had been published as a portrait and not as an insulting caricature.
It said it had contacted the Iranian embassy on Monday to resolved misunderstandings.