DAMASCUS, May 22 (Xinhua) -- The U.S. and its allied militant groups on ground in Syria are eyeing the de facto capital of the Islamic State (IS) in northern Syria as their next target, threatening the group's leader of killing him sooner or later.

Liwa Thuwar al-Raaqa, Arabic for al-Raqqa Revolutionist Brigade,  said Sunday that it was ready for a battle to liberate Syria's northern city of al-Raqqa from the IS group.

"Preparations have been made for the decisive step and we are ready with full equipment and manpower for the imminent battle by God's will," the group said on its official Twitter account.

Since early this year, Liwa Thuwar al-Raqqa has become part of the Kurdish-led Syria Democratic Force (SDF), an alliance of Kurdish fighters and other Syrian rebel groups deemed as "moderate" by the West.

The SDF has been largely supported by the U.S. in the battles against the IS over the past year.

The declared aim of the group was to defeat IS in northern Syria, and apparently stripping the terror group of its self-declared capital is their next move, especially after the recent visit of a U.S. official to northern Syria.

General Joseph Votel, head of U.S. Centcom, secretly visited northern Syria on Friday for 11 hours, during which he met with commanders of SDF and other U.S.-backed rebel groups, including the Liwa Thuwar al-Raqqa.

The visit was said to aim to coordinate the U.S.-led coalition and rebel plans in taking back Syria's northern province of Raqqa.

Pan-Arab al-Mayadeen TV said in a special report Sunday that a plan for unleashing an attack on al-Raqqa was put forward during the meeting.

According to sources familiar with the situation, the TV said the attack is expected to be launched from the towns of Ein Issa, and the countryside of the city of Ayn al-Arab, or Kobane, in the countryside of the northern province of Aleppo with the participation of 12,000 fighters.