An Iraqi Shia Muslim force backed by Iran said it pushed Islamic State out of a group of villages on the border with Syria on Monday, potentially reopening a supply route to send Iranian weapons to President Bashar al-Assad.
The manoeuvre could also be the prelude to a connection with the Iranian-backed forces of Assad, although they are yet to reach the Iraqi border from the Syrian side.
Syrian rebel sources have warned of advances by the Syrian army and Iranian-backed militia to reach the border.
The territory taken by the Popular Mobilisation force on Monday is located north of the Islamic State-held town of Baaj.
For Popular Mobilisation, it is a step towards achieving a linkup with Assad forces, giving him a significant advantage in fighting the six-year rebellion against his rule.
But the territory is connected with land held by U.S.-backed Syrian Kurdish groups on the Syrian side.
They are more focused on fighting Islamic State than Assad and it is not known whether the Syrian Kurds would allow the Iraqi Shia force to use their territory to reach Assad's troops, deployed further south and further west.
In a statement on its website, Popular Mobilisation described its advance to the border with Syria as "a Ramadan miracle", referring to the Muslim fasting month which started over the weekend.
Popular Mobilisation is taking part in the U.S-backed Iraqi campaign to defeat Islamic State in the city of Mosul and the surrounding province of Nineveh.
Iraqi government armed forces are focusing their effort on dislodging insurgents from the city of Mosul, Islamic State's de-facto capital in Iraq.
While reporting nominally to Iraq's Shia-led government, Popular Mobilisation has Iranian military advisers, one of whom died last week fighting near Baaj. Iran has helped to train and organise thousands of Shia militia fighters from Iraq, Afghanistan and Pakistan in the Syrian conflict.
Fighters from Lebanon's Hezbollah are also working closely with Iranian military commanders in Syria.