TIKRIT — Rebels from an Al-Qaeda splinter group overran the Iraqi city of Tikrit on Wednesday and closed in on the biggest oil refinery in the country, making further gains in their rapid military advance against the Shiite-led government.

The threat to the Baiji refinery comes after militants from the Islamic State in Iraq and the Levant (ISIL) seized the northern city of Mosul, advancing their aim of creating Sunni Caliphate straddling the border between Iraq and Syria.

The fall of Mosul, Iraq’s second biggest city, is a blow to Prime Minister Nouri Al-Maliki’s attempts to defeat the Sunni militants, who have regained territory in Iraq over the past year following the withdrawal of US forces, seizing Falluja and parts of Ramadi west of Baghdad at the start of the year.

An estimated 500,000 Iraqis have fled Mosul, home to 2 million people, and the surrounding province, many seeking safety in the autonomous Kurdistan region.

Security sources said ISIL militants on Wednesday drove more than 60 vehicles into Tikrit, the hometown of former Iraqi leader Saddam Hussein, occupying local government buildings and raising ISIL’s black flag overhead.

“I was in a security station in Tikrit. There were three policemen with me and we were taken by surprise when militants started shooting machine guns and speaking on loudspeakers, telling us to leave,” said police captain Saleh Al-Jubbouri.

“The three policemen changed their clothes and vanished.” Foreign Minister Hoshyar Zebari said Iraq’s leaders must unite to face a “mortal” threat. “There has to be a quick response to what has happened,” he said during a trip to Greece.

Zebari said Baghdad would work with forces from Kurdistan in the north to drive the fighters out of Mosul after they put Iraqi security forces to flight on Tuesday.

Maliki described the fall of Mosul as a “conspiracy” and said those who had abandoned their posts would be punished. He also said Iraqis were volunteering in several provinces to join army brigades to fight ISIL.

In a statement on its Twitter account, ISIL said it had taken Mosul as part of a plan “to conquer the entire state and cleanse it from the apostates”, referring to the state of Nineveh of which the city is the capital.

ISIL’s rapid advances show that Iraq’s security forces — trained and equipped by Washington at a cost of nearly $25 billion and numbering more than a million strong — are outmatched against foes who once took on the full might of the US.

Overnight on Tuesday, ISIL militants moved on Baiji, home to Iraq’s largest refinery, which can process 300,000 barrels per day and supplies oil products to most of Iraq’s provinces and as well as Baghdad. Security sources said the fighters drove into the town of Baiji in armed vehicles, torching the courthouse and police station before freeing prisoners.

Local officials and residents said they withdrew on Wednesday into the surrounding villages after local tribal leaders persuaded ISIL not to take over the energy installations in Baiji, including the refinery and power stations.

Displacement

The governor of Mosul blamed Maliki for failing to act upon his warnings about the threat of ISIL.

“The entry of ISIL to Mosul was through the desert from Syria,” Atheel Al-Nujaifi said. “There are camps in the desert and we have repeatedly asked the government to bomb these camps instead of luring ISIL into the cities to fight it.”

At a checkpoint on the road between Mosul and Arbil, residents who fled with little more than the clothes on their backs were stunned by the turn of events and did not know what to make of their city’s new occupants.

A 40-year-old man who fled the city with his family said: “We are frightened because we don’t know who they are. They told us not to be scared and that they came to liberate and free us from oppression.”