Iraq is building a concrete wall along part of its border with Syria to stop ISIS group militants from infiltrating, an Iraqi military source said Sunday.

In the "first stage" of construction, a wall around "a dozen kilometers (seven miles) long and 3.5 meters (11 feet) high was built in Nineveh province", in the Sinjar area of northwest Iraq, a senior officer told AFP, requesting anonymity.

Iraq, which shares a more than 600-kilometer long border with Syria, seeks to "put a stop to the infiltration of ISIS members" into its territory, the source added, without specifying how long the wall would eventually run.

Iraq in 2018 said it had begun building a fence along the Syrian border for the same reason.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said the wall's construction was carried out in an area facing the town of Al-Shaddadi, in the south of Syria's Hasakeh province.

In January in the Kurdish-controlled province, ISIS fighters attacked a prison to free fellow militants, sparking days of clashes that left hundreds dead.

Many prisoners are thought to have escaped, with some crossing to neighboring Turkey or Turkish-held territory in Syria's north, the Observatory said.

ISIS overran large swathes of Iraq and Syria in 2014, declaring a "caliphate" before Baghdad proclaimed victory in late 2017 after a grinding campaign.

But a low-level insurgency has persisted, flaring up particularly in rural and mountainous areas between Iraq's autonomous Kurdistan region and northern outskirts of the capital.