Syrian Democratic Forces capture a dozen villages near Raqqa

Syrian Kurdish-led forces are continuing their advance on the Islamic State group "capital" Raqqa capturing more ground in eastern Syria on Tuesday.

US-backed Syrian Democratic Forces seized nine villages on Monday, bringing the total to 12 settlements captured in the past 36 hours.

Kurdish fighters captured Tal Othman later on Tuesday, a village on the banks of the Euphrates and south of the Tishreen Dam.

"The SDF has captured 12 villages... northwest of Raqqa in the past 36 hours," Syrian Observatory for Human Rights chief Rami Abdel Rahman told AFP.

The villages lie 80 kilometres (50 miles) or more from Raqqa city although capturing IS' de-facto capital appears to be a distant aim for the group, which still have vast swathes of territory to capture from the group first.

The target of the current offensive appears to be the town of Tabqa and its adjacent dam on the Euphrates River, which lie some 40 kilometres (25 miles) upstream.

The Syrian Observatory says IS lost 18 fighters in the fighting on Monday, with 79 lost in total since the start of the offensive on May 24.

There was no confimation of the number of SDF losses, but there have been reports of an Australian foreign fighter with a Kurdish unit among the casualties.

Saudi Arabia's Foreign Minister Adel al-Jubeir said his troops were ready to go to Syria as part of an international coalition, and reiterated his call for the world to take action against Bashar al-Assad

The United States has been taking a front seat role in the offensive, deploying 200 special forces to work alongside the anti-IS coalition.

US-led coalition air strikes have also helped puncture IS' defences allowing the brigades to capture new territories.

The situation of US forces working alongside these fighters has angered its Nato ally Turkey. It views the Kurdish People's Protection Units - which dominates the Syrian Democratic Forces - as a branch of the banned Kurdistan Workers Party [PKK], which has waged a three-decade insurgency against the Turkish army.

On Sunday, Turkish Foreign Minister Mevlut Cavusoglu said Ankara was ready to join forces with Washington for a special operation inside Syria. However, he said that the force would have to exclude the Kurdish forces, which have become the driving force of the current anti-IS offensive.

Meanwhile, Saudi Arabia's Foreign Minister Adel al-Jubeir again said his troops were ready to go to Syria as part of an international coalition, and reiterated his call for the world to take action against President Bashar al-Assad.

Agencies contributed to this story.