In contrast to the Iraqi army's failures, Kurdish fighters in Syria are on the march against the Islamic State group, capturing towns and villages in an oil-rich swath of the country's northeast under the cover of U.S.-led airstrikes.

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 — so clearly absent in Iraq.

In Syria, a country now split mostly between al-Qaida-style militants and forces loyal to President Bashar Assad, the U.S. has found a reliable partner in the country's main Kurdish fighting force, known as the YPG. They are moderate, mostly secular fighters, driven by revolutionary fervor and deep conviction in their cause.

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 IS. Along the way, they have picked up ammunition, weapons and vehicles left behind by Islamic State 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 Islamic State group's de-facto capital in its self-declared caliphate.

"The YPG doesn't lack a will to fight, like soldiers in the Syrian army, or soldiers in the Iraqi army who mostly fight for a salary," said Wladimir van Wilgenburg, a Middle East analyst at the Jamestown Foundation in Washington. "The YPG is much more motivated than other forces in the region, and doesn't lack cohesion and doesn't have coordination problems."

"The YPG is quite ideological, while the Syrian and Iraqi army aren't very well motivated," van Wilgenburg added.

The Iraqi military has struggled to make gains after its humiliating defeats last year, when it virtually crumbled in the face of the militant onslaught in northern Iraq. Poor training, corruption and sectarian politics have all been cited as reasons for the military's shortcomings. The U.S. spent billions of dollars training Iraqi forces from 2003 to 2011, but much of that training did not reach the foot soldiers battling the Islamic State group today.

Last week, U.S. Defense Secretary Ash Carter criticized the Iraqi army following the fall of Ramadi, the strategic capital of the country's largest Sunni province of Anbar, saying the Iraqi military lacked the "will to fight."

Within days, IS fighters had also dealt a major blow to Assad's forces in Syria, seizing the historic city of Palmyra, a major crossroads linking the capital, Damascus, with territory to the east and west. In images reminiscent of the Iraqi defeat in Ramadi, Syrian soldiers fled the city, leaving behind tanks and ammunition.

By contrast, Syria's Kurds have shown remarkable cohesiveness. Spurred by ideology and nationalistic fervor, they fought ferociously to claw back territory after Islamic State militants captured the Kurdish town of Kobani and dozens of surrounding villages last fall. The IS descent on Kobani led the U.S. to widen its air campaign from Iraq into Syria to assist the Kurds.

Despite the fact that IS had heavy weapons and far more fighters, the Kurds resisted the offensive on Kobani, conducting fierce street battles until the U.S. air-dropped weapons and intensified the airstrikes.

In January, the YPG liberated Kobani from the Islamic State militants and began a wide offensive in which they regained much of the territory they had lost.

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, said Ghalia Nehme, a member of the Kurdish Women's Protection Units.

"The airstrikes were a major help for the fast advance," Nehme said, adding that 425 square miles (1,100 square kilometers) were taken from IS over the past three weeks alone.

Nasser Haj Mansour, a defense official in Syria's Kurdish region, said the YPG is in close contact with the U.S.-led coalition and sometimes asks for airstrikes targeting IS positions after its fighters locate them.

"Warplanes are not enough to liberate a region, but without the air force it will be more difficult," said Haj Mansour. "The push won't be as easy and fast."

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.

Such a move is likely to anger Turkey, which sees the YPG as part of the Kurdish PKK movement that has waged an anti-government insurgency in southeastern 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 IS.

The latest village to fall was Mabrouka on Tuesday, bringing the Kurdish force closer to Tel Abyad. The fighting has killed 532 Islamic State militants, the YPG said.

The Kurds are backed by Arab tribesmen, Assyrian Christian gunmen and members of the rebel faction known as Burkan al-Furat — Arabic for the "Volcano of the Euphrates."

U.S.-led coalition airstrikes have concentrated on areas around Kobani and Hassakeh, only rarely hitting Raqqa or other Islamic State-held areas in the Aleppo countryside.

But in mid-May activists said the U.S.-led coalition dropped leaflets on Raqqa with this warning in Arabic: "We killed many of your commanders and fighters. We can carry out raids against you anytime and anywhere, and you don't have the power to stop us and we will not stop."

"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.

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Associated Press writer Vivian Salama in Baghdad contributed to this report.