Iraq's prime minister has asked parliament to declare a state of emergency, after Islamist militants effectively took control of Mosul.
Nouri Maliki acknowledged "vital areas" of the northern city had been seized.
Overnight, hundreds of armed men seized local government's offices and police stations before taking control of the airport and the army's headquarters.
About 150,000 people are thought to have fled and authorities have set up camps for them in three nearby towns.
Elsewhere, a double bomb attack in the central town of Baquba killed at least 20 people, police and medics said. The blasts, targeting a funeral procession, also wounded 28 people.
In the past week, the jihadist Islamic State in Iraq and the Levant (ISIS) and its allies have carried out major attacks on cities and towns in western and northern Iraq, killing scores of people.
The BBC's Jim Muir in Beirut says militants from ISIS have been informally controlling much of Nineveh province for months, imposing tolls on the movement of goods and demanding protection money from local officials.
After five days of fighting, they took control of key installations in Mosul, Iraq's second largest city with an estimated population of 1.8 million.
On Monday, Nineveh Governor Atheel al-Nujaifi made a televised plea to the city's residents, calling on them to "stand firm in their areas and to defend them against the strangers".
But Nujaifi fled shortly before the provincial government's headquarters fell to the onslaught late on Monday night.
On Tuesday, several residents told the Associated Press that black flags associated with jihadist groups were flying from buildings and that the militants had announced over loudspeaker that they had "come to liberate Mosul and would fight only those who attack them".
"The situation is chaotic inside the city and there is nobody to help us," said Umm Karam, a government employee. "We are afraid."
Many police stations were reported to have been set on fire - local TV stations showed pictures of plumes of smoke rising over the city - and hundreds of detainees set free.
Osama al-Nujaifi, the speaker of parliament and brother of Nineveh's governor, called on the Iraqi government and Kurdistan Regional Government to send reinforcements to Mosul to "fight the terrorists", whom he said had seized military hardware, including helicopters.
"What happened is a disaster by any standard," he said. "The presence of these terrorist groups in this vast province... threatens not just the security and the unity of Iraq, but the whole Middle East."
Later, Maliki told a televised news conference in Baghdad that the security forces had been placed on a state of "maximum alert".
He also said he had asked parliament to declare a state of emergency, which would broaden arrest powers and allow curfews to be imposed.
Meanwhile, the Turkish consulate in Mosul confirmed reports that 28 Turkish lorry drivers had been abducted by militants in Nineveh.
Our correspondent says the loss of Mosul is a massive challenge to Maliki. The mainly Sunni city has long chafed under his rule, he adds.
It comes as the Iraqi government struggles with a surge in sectarian violence that killed almost 800 people, including 603 civilians, in May alone, according to the UN. Last year, more than 8,860 people died.
Parts of Ramadi, the capital of Anbar province, and much of the nearby city of Falluja have been under the control of ISIS and its allies since late December, something that Maliki has been unable to reverse.