BAGHDAD — Iraq’s Shi’ite paramilitary forces reached the border with Syria on Monday after securing a string of small villages west of Mosul, according to a spokesman for the group.

The move follows a push by the government-sanctioned forces to retake a number of small villages and key supply lines from the Islamic State in the vast deserts west of Mosul. Iraq’s conventional military has focused on clearing the city itself, a slow, grinding process in dense urban terrain packed with tens of thousands of civilians.

The paramilitary forces — mostly Shi’ite fighters with close ties to Iran referred to as the Popular Mobilization Forces — began Monday’s operation by pushing Islamic State militants out of the center of the town of Baaj, some 25 miles from the Syrian border, according to Shi’ite lawmaker Karim al-Nouri.

The fighters plan to ‘‘erect a dirt barricade and dig a trench’’ along the border, said Sheikh Sami al-Masoudi, a PMF leader, describing how the forces would secure the porous border area that has long been a haven for smugglers and insurgent activity.

Masoudi described Baaj as the last Islamic State supply line between Syria and Iraq in the area and said the paramilitary forces reached the border by retaking the village of Um Jrais.

Iraq’s army, federal police, and special forces launched the operation to retake Mosul last October with close backing from the US-led international coalition.

The city’s eastern half was declared liberated in January, and the push for the city’s western section, separated from the east by the Tigris River, began the following month.

Iraqi forces are closing in on the last pockets of Islamic State control in Mosul’s Old City, a slow, grueling fight that the United Nations warns could put the more than 100,000 civilians still trapped by the Islamic State at severe risk.

Meanwhile, the Iran-backed PMF — known as Hashed al-Shaabi in Arabic — has largely operated in the desert to the west of Mosul, retaking small villages and cutting Islamic State supply lines since October.

After securing the border area, Iraq’s Popular Mobilization Forces are ready to move inside Syrian territories, according to Hashim al-Mousawi, a leader with the powerful al-Nujaba militia that falls under the PMF umbrella.

But, Mousawi added such a move would require the approval of the Iraqi government.

On the Syrian side of the border, US-backed Syrian Democratic Forces, Islamic State militants, and Syrian rebels are fighting for territory. President Bashar Assad’s forces and their allies have also been on the offensive, moving toward the Iraqi and Jordanian border but are still far from reaching it.

Associated Press