Militants Sweeping Toward Baghdad / The New York Times

By late Wednesday there were unconfirmed reports that the Sunni militants, many aligned with the radical Islamic State of Iraq and Syria, were battling loyalist forces at the northern entrance to the city of Samarra, about 70 miles north of Baghdad. The city is known for a sacred Shiite shrine that was bombed in 2006, during the height of the American-led occupation, touching off sectarian mayhem between the Sunni minority and Shiite majority.Sunni Militants Drive Iraqi Army Out of MosulJUNE 10, 2014

An influential Iraqi Shiite cleric, Moktada al-Sadr, called for the formation of a special force to defend religious sites in Iraq. The authorities in neighboring Iran, which is predominantly Shiite, canceled all visas and flights for pilgrims to Baghdad and intensified border security, Iran’s official Islamic Republic News Agency reported.

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The remarkably fast advance of the Sunni militants, who now control strategically important swaths of northern and western Iraq, reflects the spillover of the Sunni insurgency in Syria and the inability of Iraq’s Shiite-led government to pacify the country in the more than two years since American forces departed after eight years of war and occupation.

Insurgents also raided the Turkish consulate in Mosul and seized the consul general and 47 other Turkish citizens, including special-forces soldiers and three children of diplomats, the Turkish prime minister’s office said. The development raised the possibility that Turkey, a NATO ally that borders both Syria and Iraq, would become directly entangled in the fast-moving crisis.

Prime Minister Recep Tayyip Erdogan of Turkey was holding an emergency meeting with top security officials on Wednesday to discuss the crisis, and the Turkish foreign minister cut short a trip to New York and was returning to Ankara, government statements said.

Turkey has long taken an interest in northern Iraq for economic reasons and because of the sizable and often restive Kurdish minority, which straddles the border and controls a region of Iraq east of Mosul.

Amid the collapse of the Iraqi army in Mosul, Tikrit and other northern cities, questions began to be raised about the possibility of a conspiracy in the military to deliberately surrender. Witnesses reported some remarkable scenes in Tikrit, where soldiers handed over their weapons and uniforms peacefully to militants who ordinarily would have been expected to kill government soldiers on the spot.

Prime Minister Nuri Kamal al-Maliki, a Shiite, himself suggested the possibility of a disloyal military himself in his exhortations on Tuesday for citizens to take up arms against the Sunni insurgents.

Citizens in Baiji, a city of 200,000 about 110 miles south of Mosul, awoke Wednesday to find that government checkpoints had been abandoned and that insurgents, arriving in a column of 60 vehicles, were taking control of parts of the city without firing a shot, the security officials said. Peter Bouckaert, the emergency services director for Human Rights Watch, said in a post on Twitter that the militants had seized the Baiji power station, which supplies electricity to Baghdad, Kirkuk and Salahuddin Province.

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In Tikrit, famous as the hometown of Saddam Hussein, residents said the militants attacked in the afternoon from three directions: east, west and north. Residents said there were brief exchanges of gunfire, and then police officers and soldiers shed their uniforms, put on civilian clothing and fled through residential areas to avoid the militants, while others gave up their weapons and uniforms willingly.

The militants’ advance spread alarm in Baghdad, 110 miles south. Though the city seemed calm, residents said they were shocked by the news and feared that the Islamic State of Iraq and Syria would push on toward the capital.

Shiite militias and security forces loyal to the Shiite-led government of Prime Minister Maliki were on high alert, and residents in Baghdad began stockpiling food, fuel and small arms in fear of a rebel assault. A senior provincial official said the authorities had a plan to recapture Mosul, according to news agency reports, but it was unclear how.

On Wednesday, the insurgents claimed to have taken control of the entire province of Nineveh, Agence France-Presse reported, and there were reports of militants executing government soldiers in the Kirkuk region. Atheel al-Nujaifi, the governor of the province, criticized the Iraqi army commanders in Mosul, saying they had misled the government about the situation in the city.

Iraq’s foreign minister, Hoshyar Zebari, was quoted on Wednesday as saying his country’s Kurdish minority would “work together” with Baghdad’s forces to “flush out these foreign fighters.”

At a meeting of Arab and European foreign ministers in Athens, Mr. Zebari, himself a Kurd, called the insurgents’ strike “a serious, mortal threat,” adding: “The response has to be soon. There has to be a quick response to what has happened.”

Iraqi Kurds are concentrated in the autonomous region of Kurdistan, where security is maintained by a disciplined and fiercely loyal fighting force, the pesh merga, that has not yet become involved in the latest clashes.

In a further indication of the regional dimensions of the crisis, the government of President Bashar al-Assad in Syria, facing the same jihadist adversary in its civil war against a broader array of armed foes, expressed solidarity with the Iraqi authorities and armed forces, the official SANA news agency reported.

Word of the latest militant advance came as a United Nations agency reported that 500,000 people had fled Mosul — Iraq’s second-largest city, with a population of about 2 million — after the militants, spilling over the border from Syria, captured military bases, police stations, banks and provincial headquarters.

The International Organization for Migration, based in Geneva, said the civilians had mainly fled on foot, because the militants would not let them use vehicles and had taken control of the airport. Roughly the same number were displaced from Anbar Province in western Iraq as the militants gained ground there, the organization said.

On Tuesday the insurgents, reinforced with captured weaponry abandoned by the fleeing government forces, raised their black banner over streets in Mosul littered with the bodies of soldiers, police officers and civilians. The success of the militant attack was the most stunning development in a rapidly widening insurgency straddling the porous border of Iraq and Syria.

Mr. Maliki has ordered a state of emergency for the entire country and called on friendly governments for assistance in a quickly deteriorating situation. His weak central government is struggling to mount a defense, a problem made markedly more dangerous by the defections of hundreds of trained soldiers and the loss of their vehicles, uniforms and weapons.

Security officials said the militant drive toward Baiji began late on Tuesday with brief clashes a few miles north of the town before the insurgents overran a security post, captured vehicles and set buildings on fire.

“They did not kill the soldiers or policemen who handed over their weapons, uniform and their military I.D.,” a security official in Tikrit said on Wednesday before the militants reached that city; he spoke on the condition of anonymity. “They just took these things and asked them to leave,” the official said.

The swift advances offered a new milestone in Iraq’s unraveling since the withdrawal of American forces at the end of 2011.

The rising insurgency also presented a new quandary for the Obama administration, which has faced sharp criticism for its recent swap of five Taliban officers for Sgt. Bowe Bergdahl and must now answer questions about the death of five Americans by friendly fire in Afghanistan on Monday night.

Critics have long contended that America’s withdrawal of troops from Iraq, without leaving even a token force, invited an insurgent revival.