As the Kurds close in on Tel Abyad, a major commercial center on the Turkish border, their advance highlights the decisive importance of combining airstrikes with the presence of a cohesive and motivated ally on the ground.

Since the beginning of May, they have wrested back more than 200 Kurdish and Christian towns in northeastern Syria, as well as strategic mountains seized earlier by ISIS. Along the way, they have picked up ammunition, weapons and vehicles left behind by ISIS fighters.

The push has gotten them closer to Tel Abyad, a major avenue for commerce for the extremist group through which it smuggles foreign fighters and sells black-market oil to help fund its conquests. The city is also a key link between Turkey and the northern Syrian city of Raqqa, the ISIS terrorist group's de-facto capital in its self-declared caliphate.

Syria's Kurds have shown remarkable cohesiveness. They fought ferociously to claw back territory after ISIS militants captured the Kurdish town of Kobani and dozens of surrounding villages last fall.

On May 6, the Kurdish fighters and their allies launched an offensive from the northeastern province of Hassakeh and in less than three weeks captured the strategic Abdul-Aziz Mountain along with 221 Kurdish and Christian villages that were held by IS.

The mountain had been a key militant base for sending vehicles rigged with explosives into Kurdish areas.

The Kurds are now closing in on Tel Abyad with fighters moving east from Kobani and west from Hassakeh — an offensive that, if successful, would open a direct line between Kurdish-controlled territory along the border with Turkey.

"Tel Abyad is going to be the next target. We want to strangle Daesh," said Nawaf Khalil, a Kurdish official who currently heads the Europe-based Center for Kurdish Studies, using an Arabic acronym for ISIS.

"There is no doubt that the liberation of Tel Abyad, will be the pass for the liberation of Raqqa," said Idriss Naasan, a Kurdish official in Kobani.

The Islamic State group's leader, Abu Bakr al-Baghdadi, indicated as much in an audio message released earlier this month.

"The battle of Raqqa will come before the battle of Mosul," he said, referring to the Islamic State's stronghold in Iraq, which Iraqi forces have been preparing a major offensive to try to retake.