This post was updated at 10:30 p.m. ET

As refugees stream out of Mosul after the Iraqi city was captured by forces of the al-Qaida-linked Islamic State of Iraq and Syria, NPR's Deborah Amos passes along reports that Tikrit, the hometown of the late dictator Saddam Hussein, has also been overrun.

The Associated Press says "soldiers and security forces [in Tikrit have] abandoned their posts and yielded ground once controlled by U.S. forces."

According to AP:

"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 municipal council of nearby Samarra."

The latest news comes amid reports that a half-million people have been streaming out of Mosul, which was seized by Islamist militants this week. ISIS militants took over much of the city after Iraqi security forces seemingly abandoned their posts.

In a statement on Wednesday, the U.N. Security Council called the moves by al-Qaida-affiliated militants in Iraq "an attempt to destabilize the country and region."

"The members of the Security Council reiterated that no act of violence or terrorism can reverse a path towards peace, democracy and reconstruction in Iraq, underpinned by the rule of law and respect for human rights, which is supported by the people and the Government of Iraq and the international community," the Security Council said.

This morning, hundreds of people were filing past checkpoints on the road from Mosul to reach northern Iraq — a sight witnessed by NPR's Alice Fordham, who reported on the takeover for Morning Edition.

While ISIS has previously taken over smaller territories and towns in Iraq, Mosul is a hub of commercial activity and has a population of about 2 million. It was a hotly contested city during the U.S.-led invasion of Iraq.

"This is a really key, strategic city," Fordham says, "and I don't think anyone was expecting it to fall so fast, so completely."

There are reports that the ISIS fighters might now be heading toward Baghdad. The group might also gain control of a crucial oil refinery.

A report by the International Organization for Migration says an estimated 500,000 people have fled Mosul since hostilities began Saturday morning. The Iraqi security forces left the city Monday.

Iraqi Prime Minister Nouri al-Maliki "has asked Parliament to declare a state of emergency," the BBC reports.

"This is a hugely symbolic victory for the most extreme, violent and sectarian terrorist group in the Arab world today," Bruce Riedel of the Brookings Institution tells NPR's Amos.

Riedel says the takeover raises big questions about the U.S.-trained Iraqi military, and that any response isn't as straightforward as building infrastructure or training soldiers.

"It's not a question of giving them airplanes or Humvees," he says. "It's a question of building a political state to which they feel some kind of loyalty. America can provide Humvees. America can't build a state for them."

A statement from the White House press secretary Wednesday night said the U.S. "will stand with Iraqi leaders across the political spectrum as they forge the national unity necessary to succeed in the fight against ISIL."

Noting that Mosul is Iraq's third-largest city, the Christian Science Monitor reports that "the scale of the catastrophe, as troops loyal to Mr. Maliki flood north and troops controlled by the Kurdish Regional Government rush west and south, can't be overstated. Chicago is the United States' third-largest city. Munich is Germany's. Osaka is Japan's."

The figure of 500,000 people displaced in Mosul eclipses the 480,000 people that the U.N. recently estimated had fled Iraq's Anbar province in the first six months of this year.

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