Story highlights ISIS now controls Al-Qaim, Rawa, Ana and Husaybah

Those four towns sit on a highway from Syria to Baghdad

Several Sunni tribes are supporting ISIS in Anbar province

Husaybah is just 62 miles, or 100 kilometers, outside Baghdad

A strategic border crossing and three other towns in western Iraq fell Saturday to the control of ISIS militants, a senior Iraqi security official said.

In addition to their offensives in northern Iraq, the militants have now strengthened their hand in the western province of Anbar, the country's largest geographically, and were controlling Al-Qaim, Rawa, Ana and Husaybah, said the senior official, who's based in Anbar.

Most importantly, the Islamic State in Syria and Iraq, or ISIS, controlled the strategic town of Al-Qaim on the border with Syria, where the enemy fighters enjoy a stronghold, Iraqi security officials said Saturday.

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Together, the four towns are situated along a highway from Syria to Baghdad, heightening possibilities that the militants could now march from the west to lay siege to the Iraqi capital. One of the four towns, Husaybah, is just 100 kilometers, or 62 miles, outside Baghdad.

CNN's Nic Robertson says fighters from Syria are capable of reaching the outskirts of Baghdad in less than four hours.

Iraqi government officials didn't have an immediate comment, other than security officials saying they were expecting troop reinforcements in Anbar.

Several Sunni tribes are aiding and supporting ISIS in Anbar, the senior official said.

Also, the first retinue of U.S. military advisers was expected to arrive soon in Iraq.

Since clashes erupted Friday in Al-Qaim, at least 11 Iraqi soldiers have been killed and 21 more have been wounded. Also, at least 20 militants were killed after Iraqi forces shelled areas from where the extremists launched attacks, two security officials in Ramadi, Iraq, told CNN.

Al-Qaim sits across from Syria's Deir Ezzor province, where ISIS controls at least three towns, including areas near the military airport of Deir Ezzor, which was the headquarters of the military council for rebel battalions, said the Syrian Observatory for Human Rights, an opposition group in London that monitors the Syrian conflict.

Why Al-Qaim is important

"This advancement (in Syria) is considered a very important and strategic step because ISIS has tried to take a complete control over areas in the east of Deir Ezzor in order to reach to the Syrian-Iraqi borders, and then to connect its held areas in both Syria and Iraq with each other," the opposition group said.

Opposite of Al-Qaim is the Syrian town of Al-Bukamal, which is under the control of other Islamist brigades such as Nusra Front, said Rami Abdulrahman of SOHR. ISIS doesn't control that town, he added Saturday.

Iraqi forces were fighting the suspected ISIS militants on at least two fronts: First, they discovered dozens of militants on the Syrian side of the border, security officials said.

At the same time, the support by some Sunni tribesmen for ISIS is proving pivotal in the militants' success, a senior security official in Ramadi told CNN.

If the Sunni tribes do not decide to help and support Iraqi security forces, then it will be very difficult for Iraqi forces to regain the full control of Al-Qaim, the senior official said.

In the meantime, Iraqi forces were waiting for more troops to arrive in Al-Qaim, located about 500 kilometers (about 310 miles) west of Baghdad.

Shiite show of force

Iraq has a long history of brutality between Sunni and Shiite sects, and on Saturday, a warning of renewed conflict between Shiites and the Sunnis supporting ISIS emerged on the streets of Baghdad, where thousands of Shiite militiamen marched in a rally.

The show of force, called a parade, was organized by prominent Shiite cleric Muqtada al-Sadr, who urged unity in Iraq in its fight against ISIS.

The thousands of Shiites wore various security forces uniforms in a march in Baghdad's Sadr City.

They carried rifles, rocket launchers and rocket-propelled grenades.

On the street, they also did a demonstration on planting roadside bombs -- with the armor-piercing bombs hoisted on their shoulders. Those bombs were the Iranian-designed, electronically formed projectiles, which were the scourge of the U.S. war in Iraq during the 2000s.

Even women joined the march, dressed in black and holding handguns.

Only a few years ago, the Shiite militiamen were fighting U.S. forces on the same Baghdad streets.

On Saturday, they demonstrated their readiness for the ISIS fighters.

"ISIS is a terrorist organization created by the United States. They are the enemy of humanity," said one participant who identified himself as a former army colonel. "We are here to free the land for all Sunni and Shiite and everyone."

Another militiaman said he didn't welcome the U.S. initiative to send military advisers to help the government.

"We don't need airstrikes or any external force helping us here," the volunteer said. "We don't want these American military advisers."

A Shiite cleric blamed Prime Minister Nuri al-Maliki for Iraq's latest instability, even though his government favors Shiites.

"The main cause of the security deterioration is the bad management of the prime minister," the cleric said. "Al-Maliki must leave and resign."

Falluja fighting, Baghdad bombings

Elsewhere in Iraq's western Anbar province, Iraqi security forces killed 15 "terrorists" and destroyed four vehicles on Saturday afternoon in Falluja, said Iraqiya State TV, citing security officials.

Falluja is about 60 kilometers west of Baghdad, and Iraqi forces have so far blocked the militants from marching on the nation's capital. Falluja has been under control of ISIS militants and Sunni tribesmen since January.

The militants in Falluja, however, have been trying to take over Sunni areas close to Baghdad, such as Abu Ghraib and small villages close by, Ramadi security officials told CNN.

Abu Ghraib is a largely Sunni area in the western outskirts of the capital.

Despite the government effort to protect Baghdad, several bomb attacks occurred across the capital city, killing at least seven people and wounding 32 more, officials said Saturday. One of the bomb attacks hit Baghdad's Sadr City, a predominantly Shiite area in the eastern part of the Iraqi capital, police officials in Baghdad told CNN Saturday.

U.S. advisers due to arrive in Iraq soon

The Iraqi government was waiting for the initial group of U.S. military advisers to arrive in Iraq soon, a senior defense official said, as crowds paraded nationwide in a show of unity for the government.

This first detail is expected to be very small, the official said. The total number of U.S. military advisers who will eventually deploy will be about 300.

In addition, some U.S. military personnel already at the U.S. Embassy in Baghdad will be reassigned and become advisers, Pentagon spokesman Rear Adm. John Kirby said.

The first group of advisers will conduct an initial assessment of Iraqi troop capabilities and of what may be needed for a larger group of U.S. advisers, including additional security measures where they may be deployed, a senior defense official said Friday.

Refugee crisis

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More than 1 million Iraqis have fled their homes this year because of conflict, the United Nations refugee agency said Friday. The number is likely to rise as Islamist militants and Iraqi security forces battle for control.

An estimated 800,000 people left Iraq's second-largest city of Mosul after it fell to fighters from ISIS, the International Committee of the Red Cross said. The city has a population of 1.6 million.

ISIS, born from an al Qaeda splinter group and supported by Sunni factions, continues its fierce advance in Iraq.

Al-Maliki's Shiite-dominated government is accused of fostering sectarian tensions by marginalizing Iraq's Sunni Arab and Kurd minorities.

U.S. President Barack Obama told CNN on Friday that U.S. military efforts are hopeless without a change in government.

"If we don't see Sunni, Shia and Kurd representation in the military command structure, if we don't see Sunni, Shia and Kurd political support for what we're doing, we won't do it," he said.

The complete interview will be aired Monday on CNN's "New Day."

The United States withdrew its final troops from Iraq in 2011, nearly nine years after leading the invasion that ousted longtime leader Saddam Hussein.