An underground tunnel dug near the Syrian-Turkish border by the Islamic State (Isis) group has been discovered by Kurdish forces that are advancing deeper into jihadist-held territory.

Photos of the 400m-long tunnel were shared online after Kurdish People's Protection Units (YPG) found it in the wake of the fall of the border town of Tal Abyad, which was freed from IS last week.

The underground passage was half-completed with two separate ends located within Syrian territory.

It was not clear whether the Sunni extremists planned to extend the tunnel into Turkey, possibly to use it for smuggling people and goods.

Meanwhile, the YPG advanced further south, pushing the main IS defence lines on the outskirts of their de facto capital of Raqqa.

Kurdish-led forces backed by coalition airstrikes said they took over a Syrian army base south-west of of the city of Ain Issa .

"They have been defeated," YPG spokesman Redur Xelil told Reuters.

The military camp known as Liwa 93 was previously controlled by IS fighters that seized it from the regime of Bashar al-Assad last year. YPG forces were reportedly closing in onto Ain Issa, which is only 50km from Raqqa.

The Syrian Observatory for Human Rights (SOHR) said that if the town was to be conquered "the first defence lines of IS are going to be on the outskirts of the city of Raqqa".

"This means that the Islamic State keeps collapsing inside its own stronghold," Rami Abdulrahman, the head of the London-based monitoring group told Reuters.