Fighters of the Islamic State of Iraq and Syria, or ISIS, stand guard at a checkpoint in the northern Iraq city of Mosul, June 11, 2014. Since Tuesday, ISIS fighters have seized Iraq's second biggest city Mosul and Tikrit, home town of former dictator Saddam Hussein, as well as other towns and cities north of Baghdad. They continued their lightning advance on Thursday, moving into towns just an hour's drive from the capital. Reuters

The crisis in Iraq escalates hour by hour as militants from the Islamic State of Iraq and Syria, an al Qaeda offshoot, advance south and east, with their sights on Baghdad. The stunning success of the group, which seeks to establish a Sunni Islamic caliphate across Syria and Iraq, has alarmed the world due to its potential to topple the government in Baghdad, ignite a larger Sunni-Shiite conflict and even redraw the map of the entire Middle East.

In just the last two days, hundreds of thousands of Iraqis have overnight become refugees and scores of others have been killed. On Wednesday, the Sunni militants, many of them former officers in Saddam Hussein’s army, seized the city of Tikrit, the late dictator's hometown, with little trouble. The day before, they took control of Mosul, Iraq’s second-largest city, as the Iraqi army fled in panic, leaving behind its U.S.-supplied weapons, vehicles and uniforms.

The U.N. Security Council has condemned the attacks and said the humanitarian situation is “dire and is worsening by the moment.”

Here are four ways the crisis in Iraq could get even worse:

ISIS Takes Control of Baghdad: In an audio recording released on Thursday by ISIS, the militia promised to continue its city-by-city takeover, including storming the capital.

If ISIS can get close to Baghdad, the group could link up with other Sunni insurgents on the city’s perimeter. The takeover of the city would be a shattering blow to the Shi'ite-led Iraqi government of Premier Nouri al-Maliki and would likely multiply the number of civilian casualties.

U.S. Attacks With Air Strikes: Washington has called the crisis “extremely urgent” and is considering supporting the Iraq government with airstrikes against the radical Islamist militants. But that would mean intervening in another conflict not directly tied to U.S. concerns, likely mobilizing other extremist groups and further destabilizing the entire region.

The U.S. has not been active in Iraq since 2011, but a strategic agreement signed by the U.S. and Iraq in 2008 allows for limited action by American forces.

But the White House said that air strikes are not its top priority.

"While the national security team always looks at a range of options, the current focus of our discussions with the government of Iraq and our policy considerations is to build the capacity of the Iraqis to successfully confront and deal with the threat posed by [ISIS]," National Security Council spokeswoman Bernadette Meehan told The Telegraph.

The most likely U.S. intervention would be to provide military and intelligence assistance.

Iran Intervenes: Officials of the Islamic Republic, which is ruled by Shi'a clerics, have stated that they would consider intervening to “protect Shi'a shrines and cities,” Iran’s police chief said. Such an intervention could inflame tensions in the entire region, further fueling Sunni-Shi'a hostilities in Lebanon, Syria, Pakistan, Kuwait and other Muslim countries around the world.

Iran has a troubled history with Sunni extremists in its own country and could seize the opportunity to help Iraq, where Maliki is seen as an ally, strike back against a common foe.

"This is an extremist, terrorist group that is acting savagely," Iranian President Hassan Rouhani said live on state television. He added that Tehran will not "tolerate this violence and terror."

Kurds Use Crisis To Leverage Control Of Oil Sector: After government troops abandoned their posts in Kirkuk, a strategic, ethnically mixed oil city in northern Iraq, Kurdish military units stepped in, which could inflame tensions between the autonomous Kurds and Baghdad over control of the lucrative energy sector.

Kurds are the major ethnic group in northeastern Iraq, where they control an autonomous region, and they have long sought to take over Kirkuk and its enormous oil reserves.

Below is a map of where ISIS controls in Iraq: