Last Updated Jun 11, 2014 9:56 AM EDT
BAGHDAD -- Iraqi authorities are determined to recapture the northern city of Mosul after most of it was overrun by al Qaeda-inspired militants, the provincial governor said Wednesday, after himself fleeing from the city.
"Mosul is capable of getting back on its feet and getting rid of all the outsiders ... and we have a plan to restore security," said Atheel al-Nujaifi, the Ninevah provincial governor.
The stunning assault by the al Qaeda splinter group, which started on Sunday night, saw black banner-waving insurgents raid government buildings, pushing out security forces and capturing military vehicles as thousands fled for their lives from what is Iraq's second-largest city.
There were no immediate estimates on how many people were killed in the assault but the rampage by the militants sent an estimated 500,000 people fleeing from the city, according to the International Organization for Migration.
Reports suggested ISIS or other Sunni Islamic extremist factions were attacking government security forces in Tikrit on Wednesday. If confirmed, it would represent a dangerous southward advance by the militants. Tikrit is just 95 miles north of Baghdad -- roughly half-way between the capital city and Mosul.
Several Mosul residents said gunmen were knocking on their doors, meanwhile, trying to reassure locals they would not be harmed and urging civil servants to return to work. The situation appeared calm but tense, said the residents, who spoke on condition of anonymity fearing for their own safety.
In an eastern section of the city, 34-year-old Ali Sameer said mosques in his neighborhood were calling on people to return to work, especially those in public services.
Mosul's fall was a heavy defeat for Prime Minister Nouri al-Maliki amid a widening insurgency by the Islamic State of Iraq and al Sham (ISIS, also known as ISIL).
The group has been advancing in both Iraq and neighboring Syria, where they've carried out summary executions in the name of Islamic law and seized control of large swathes of territory in the country's north.
CBS News correspondent Holly Williams reports that even al Qaeda has condemned ISIS for its excessive violence, and severed ties with the group late last year.
The group's fighters come from across the Middle East and Asia, but also include hundreds of European Muslims lured into the fight by online propaganda and the call to jihad. A rival Islamic extremist militia fighting in Syria, al Nusra, maintains a direct link to al Qaeda. Al Nusra drew a young American man into the war recently who became the first U.S. national to die as a suicide bomber in the conflict.
The goal of ISIS is to establish an Islamic state straddling the border of Iraq and Syria, and the group now moves fighters freely across that border.
Williams says the stunning victory by ISIS in Mosul is more evidence that the chaos of Syria's civil war has helped breed a new generation of Islamic extremists bent on regional power.
Officials in Damascus, meanwhile, condemned on Wednesday the militants' seizure of Mosul and said Syria was offering all the support it could to Iraq to fight their "mutual enemy."
CBS News' George Baghdadi says the Foreign Ministry released a statement of solidarity with Iraq, saying the neighboring nations were facing "a terrorist invasion that targets the unity of Iraq and aims at the destruction of its people and the infrastructure."
"As Syria reaffirms its determination to fight terrorism and defend its people, it renews its full solidarity with brotherly Iraq and announces readiness to cooperate with Iraq in order to face terrorism, our mutual enemy," said the statement.
Syrian President Bashar Assad's regime has consistently referred to the myriad rebel groups fighting to force his ouster as "terrorists," and there are many groups, including ISIS, which have played a key role in the fighting.
The U.S. government, however, is supporting what it deems to be more moderate, "vetted" rebel factions.
Al-Maliki on Tuesday pressed parliament to declare a state of emergency over the Mosul attack. Legal experts said these powers could include imposing curfews, restricting public movements and censoring the media. State TV said lawmakers could convene as early as Thursday.
Al-Nujaifi, the governor of Ninevah where Mosul is the provincial capital, also accused senior security force commanders of providing Baghdad with false information about the situation in Mosul and demanding that they should stand trial.
Speaking from the northern Kurdish city of Erbil where he took refuge after leaving Mosul, al-Nujaifi also said smaller armed groups had joined ISIS during the fight for control of the city.
Videos which surfaced online Tuesday showed members of tribal militias, including the largest tribe in the region, the Duleim, fighting against government forces.