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Signs are emerging that a top fugitive and other military officers from ousted dictator Saddam Hussein's regime are backing Al Qaeda-inspired militants in Iraq, who vowed Thursday to march against Baghdad after seizing two key northern cities.

Two senior intelligence officials told The Associated Press Thursday that an armed group led by Izzat Ibrahim al-Douri -- the late leader's former deputy who escaped the 2003 U.S.-led invasion and eluded U.S. and Iraqi forces ever since – is fighting alongside the Islamic State of Iraq and the Levant.

In Saddam's hometown of Tikrit that was overrun by militants Wednesday, witnesses said fighters raised posters of Saddam and al-Douri. The two officials spoke on condition of anonymity because they were not authorized to talk to the press.

A spokesman for the Islamic State of Iraq and the Levant – also known as the Islamic State of Iraq and Syria (ISIS) -- said in audio posted on militant websites commonly used by the group that the fighters have old scores to settle with Iraq's Shiite-led government.

In Baghdad, hundreds of young men surrounded an army recruiting center after authorities issued calls for help in the fight against the ISIS, The Guardian reports. The warnings spread tensions in the capital as residents rushed to stores to buy up goods.

The ISIS – under its former name of Al Qaeda in Iraq -- was classified as a terrorist organization by the U.S. State Department in 2004.

Meanwhile, in Iraq’s second largest city of Mosul – which the ISIS captured much of on Tuesday – militants paraded American Humvees seized from Iraqi forces who evacuated the area, according to Reuters.

The militants have gained entry to the Turkish Consulate in Mosul and are holding captive 48 people, including diplomats, police, consulate employees and three children, according to an official in the office of Turkish Prime Minister Recep Tayyip Erdogan. Turkish officials believe the hostages are safe, the source said Wednesday, speaking on condition of anonymity because he was not authorized to comment to reporters on the sensitive issue.

The militants also seized 31 Turkish truck drivers in Mosul.

The Islamic State issued a triumphalist statement declaring that it would start implementing its strict version of Shariah law in Mosul and other regions it had overrun. It said women should stay in their homes for modesty reasons, warned it would cut off the hands of thieves and told residents to attend daily prayers. It told Sunnis in the military and police to abandon their posts and "repent" or else "face only death."

"People, you have tried secular regimes ... This is now the era of the Islamic State," it proclaimed.

The militants took Tikrit after police and military forces melted away following relatively brief clashes, yielding ground once controlled by U.S. forces.

In the north, Kurdish security forces on Thursday took over an air base and other posts abandoned by the Iraqi military in ethnically mixed Kirkuk, a senior official with the Kurdish forces said. He denied they had taken over the oil-rich city.

The Washington Post reported that militants had reached the city of Samarra, approximately 80 miles north of the Iraqi capital, by nightfall Wednesday.

Skirmishes continued in several areas overnight and into Thursday. Two communities near Tikrit -- the key oil refining center of Beiji and the city of Samarra, home to a prominent Shiite shrine --- remained in government hands, according to Iraqi intelligence officials.

There were clashes and gunmen tried to take the Beiji but were repelled in a rare success for Iraqi government forces protecting an important facility, the officials said.

But they added that ISIS fighters managed to take control of two big weapons depots holding some 400,000 items, including AK-47 rifles, rockets and rocket-propelled grenades, artillery shells and mortars. A quarter of the stockpiles were quickly sent to Syria in order to help the group's comrades there, they said.

On Thursday, militants attacked an Iraqi security checkpoint in the town of Tarmiyah, 31 miles north of Baghdad, killing five troops and wounding nine, officials told the Associated Press.

The ISIS’s spokesman, Abu Mohammed al-Adnani, threatened that Sunni ISIS fighters would capture the southern Iraqi Shiite cities of Karbala and Najaf, which hold two of the holiest shrines for Shiite Muslims.

Al-Adnani also said that one of his group's top military commanders, Adnan Ismail Najm, better known as Abu Abdul-Rahman al-Bilawi al-Anbari, was killed in the recent battles in Iraq.

Al-Adnani said Najm worked closely with the former leader of Al Qaeda in Iraq, Jordanian-born Abu Musab al-Zarqawi, who was killed by U.S. troops in 2006. Najm was later detained and spent years in prison before he was set free two years ago and prepared and commanded the operations that led to the latest incursions by the group in northern and central Iraq.

After Mosul fell Tuesday, al-Maliki asked parliament to give him the "necessary powers" to run the country under a state of emergency -- something legal experts said would include powers to impose curfews, restrict public movements and censor the media.

After parliament failed to reach a quorum, Hakim al-Zamili, a lawmaker affiliated with firebrand Shiite cleric Muqtada al-Sadr, played down the apparent lack of support for the vote, saying al-Maliki already has enough power to take the necessary action.

"The problem is that soldiers are not resisting the armed groups," he said. "No soldier is ready to fire a shot against the gunmen."

The Obama administration reportedly has rebuffed calls from the Iraqi government to carry out airstrikes against the Al Qaeda-aligned militants, despite apparent pleas by al-Maliki. The administration is weighing sending additional aid, but has not specified what that might be.

ISIS, which has seized wide swaths of territory, aims to create an Islamic emirate spanning both sides of the Iraq-Syria border.

The stunning advances by the Sunni militants are a heavy defeat for al-Maliki. His Shiite-dominated political bloc came first in April 30 parliamentary elections -- the first since the U.S. military withdrawal in 2011 -- but failed to gain a majority, forcing him to try to build a governing coalition.

The capture of Mosul -- along with the fall of Tikrit and the militants' earlier seizure of the city of Fallujah and parts of Ramadi, the capital of western Anbar province -- have undone hard-fought gains against insurgents in the years following the 2003 invasion by U.S.-led forces.

There were no reliable estimates of casualties or the number of insurgents involved, though several hundred gunmen were in Tikrit and more were fighting on the outskirts, said Mizhar Fleih, the deputy head of the Samarra municipal council. An even larger number of militants likely would have been needed to secure Mosul, a much bigger city.

Without assigning direct blame, al-Maliki said a "conspiracy" led to the massive security failure that allowed militants to capture Mosul, and warned that members of the security forces who fled rather than stand up to the militants should be punished.

"We are working to solve the situation," al-Maliki said. "We are regrouping the armed forces that are in charge of clearing Ninevah from those terrorists."

The White House said in a statement that Vice President Joe Biden spoke with Erdogan and called for the safe and immediate return of the Turkish personnel and family members who were reportedly taken hostage in Mosul. Turkish state-run Anadolu Agency reported that Erdogan convened an emergency Cabinet meeting.

U.N. Secretary-General Ban Ki-moon strongly condemned the abductions and the seizure of Iraqi territory by the militants, urging "the international community to unite in showing solidarity with Iraq as it confronts this serious security challenge."

The U.N. Security Council is having closed consultations Thursday on the situation in Iraq and the status of the United Nations Assistance Mission for Iraq.

White House spokesman Josh Earnest warned that the instability was rapidly becoming a humanitarian issue requiring a coordinated response by Iraq's leaders to halt ISIS's advance and wrest territory away from insurgents.

Earnest told reporters traveling with Obama that ISIS poses a "different kind of threat" to American interests than core Al Qaeda, which had repeatedly and publicly vowed to attack U.S. soil. Still, he said the U.S. was watching the threat from ISIS "very carefully" because the group has proven itself to be violent and willing to consider attacking U.S. interests and American allies.

Fox News' Jonathan Wachtel and the Associated Press contributed to this report.