DHAKA: 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, reports CNN.

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.

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.

BDST: 0900 HRS, JUN 22, 2014