Members loyal to ISIS wave its flags as they drive around Raqqa, Syria, 29 June, 2014.Reuters

With Kurdish fighters advancing to up to 50 km from Raqqa city, which is Isis' headquarters in Syria and the 'capital' of the caliphate, the Islamic State is said to be hurriedly building concrete walls in the northern part of the city to defend their territory. 

The Kurdish People's Protection Units, also simply known as YPG, have taken control of the strategic town of Ain Issa and the Isis base 'Brigade 93' to the north of Raqqa city in the last few days, according to the Syrian Observatory for Human Rights. 

"Islamic State's defensive lines have now been pushed back to the outskirts of Raqqa city because the area between Raqqa and Ain Issa is militarily weak and they have no fortifications," Rami Abdul Rahman, head of the monitoring group, told AFP. 

Abu Ibrahim Raqqawi, who heads the anti-Isis group 'Raqqa is Being Slaughtered Silently', tweeted stating that Isis has now started building concrete walls, digging trenches and putting high barriers across northern Raqqa to defend itself from the advancing Kurdish forces. 

The YPG forces have already killed 17 Isis fighters while taking control of Ain Issa in the Raqqa province, ARA News reported. 

Ain Issa is strategically located on the connecting route from Raqqa to Isis-controlled Aleppo and Hasakah, according to this BBC report. 

In retaliation to the Kurdish offensive, Isis is said to be threatening the Kurds living in Raqqa city to leave within 24 hours, Raqqawi said on Twitter.

Isis has faced several defeats to the Kurdish fighters, who are backed by the US-led coalition airstrikes, in several parts of Iraq and Syria, such as Kobani.