In the "first stage" of construction, a wall around "a dozen kilometres (seven miles) long and 3.5 metres (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-kilometre long border with Syria, seeks to "put a stop to the infiltration of Islamic State 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, IS fighters attacked a prison to free fellow jihadists, sparking days of clashes that left hundreds dead.

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

IS 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 jihadist insurgency has persisted, flaring up particularly in rural and mountainous areas between Iraq's autonomous Kurdistan region and northern outskirts of the capital.