Still rolling.

Baghdad, Iraq (CNN) — A strategic border crossing and three other cities 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 Huseiba, 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.

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.

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

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