ISTANBUL — New alliances on the battlefield and in diplomatic circles between Turkey and Saudi Arabia have managed to drastically improve the fortunes of Syrian rebel fighters struggling to make significant gains against the armies of President Bashar al-Assad.

Armed groups that were once at violent odds have banded together following an unexpected Turkish-Saudi alliance since the January death of Saudi Arabia’s King Abdullah, and the groups are making significant gains against Assad’s forces, such as the recent takeover of the northern city of Idlib.

A mutual frustration among regional leaders with perceived American indecision and inattention to Syria has drawn Turkey and Saudi Arabia closer. It’s a development that is exactly what the Obama administration wanted to see – regional actors banding together to fight their own wars – but one that could backfire if it causes further animosity toward the U.S. for refusing to do more to help stop the conflict.

Turkey and Saudi Arabia, ideological adversaries that spent decades jockeying for influence in the Islamic world, are now working together to realize their common vision for a Syria without Assad. But in recent years, the two regional powers were divided over related issues, such as the Muslim Brotherhood. Turkey has been a champion of the pan-Arab Islamist group while Saudi Arabia considered it a terrorist group. And the two sides couldn’t be more different politically: Saudi Arabia is an absolute monarchy while Turkey was founded and remains (for now) a secular republic. So far this year, it appears the powers have put aside their differences to achieve a common goal.

“A lot of the friction has abated between Turkey and Saudi Arabia,” said Joshua Landis, director of the Center for Middle East Studies at the University of Oklahoma. Much of that friction appeared to dissolve after the death of Abdullah, whose son and successor King Salman bin Abdulaziz is intent on proving his bona fides as a hawkish monarch by way of direct military action in Yemen and proxy actions in Syria.

“The new king is willing to throw might around,” Landis said.

With Qatar acting as broker, Salman and Turkish President Recep Tayyip Erdogan came to an agreement to fund and arm opposition groups battling against Syrian regime forces, and with new success.

Despite the constant chatter regarding the friendship forged over Syria, leaders in Turkey deny there is anything new to the relationship between the countries. Turkey’s Foreign Ministry spokesman Tanju Bilic said emphatically that “there are no new elements regarding this cooperation.” He also denied that Ankara was funding Islamist groups like al-Nusra Front, which is on terror list on both the U.S. and Turkey.

Local and international media quoting Turkish officials refute Bilic’s claim and characterization. A new level of cooperation between Turkey and Saudi officials, they say, is driving the gains by rebel fighters in northern Syria, which were made possible in part by an influx of arms and fighters across the porous Turkish-Syrian border.

Western observers are concerned that the Turkish-Saudi alliance could help the wrong kind of fighters and undermine the U.S.-led effort to train and equip “moderate” Syrian rebels whom the Obama administration hopes will emerge victorious and form the basis for a post-Assad Syria. The new alliance, they argue, could inadvertently empower extremist groups like al-Nusra, which if victorious would establish a hardline anti-Western caliphate.

Landis said the alliance would not have come to pass had Washington been more engaged in Syria in recent years.

“The Americans really lost their way,” he said. For Erdogan and Salman, he said, “there was no other option on the table.”

Meanwhile, Turkish opposition leaders say they are worried that greater intervention will inevitably draw Turkey’s forces into the war. Gursel Tekin, secretary general of the main opposition Republican People's Party accused Erdogan and his ruling Justice and Development Party of bringing the country to the precipice of war, publicly calling out the leader earlier this month to “deny what I said.” Several local journalists have reported on the swelling ranks of Turkish forces at the border with neighboring Syria.

The string of victories against the Syrian regime in northern Syria earlier this spring and in Syria’s central province of Homs could alter the Syrian landscape in a troubling way for Washington. According to experts and witnesses, Turkey and Saudi Arabia are backing a hardline fighter alliance with Jaish al-Fatah, otherwise known as the Army of Conquest. This umbrella group includes al-Nusra, which to Washington is akin to directly financing al-Qaeda. Another hardline group under the new umbrella is Ahar al-Sham. This new front making gains in northern Syria also includes groups that claim to benefit from the covert back in backing of the CIA such as Fursan al-Haq, or Knights of Justice brigade.

“The Americans are terrified that al-Nusra Front and it’s allies will win inside Syria,” said Fawaz Gerges, professor of international relations at the London School of Economics and former director of the school’s Middle East Centre.

“The Americans want Assad out, but not by military defeat,” Gerges said, adding that the U.S. would want Syria to maintain the structures of legitimate government institutions that might one day serve as the foundation for an elected government if and when the violence subsides.

Others contend that any way of ending the violence sooner is preferable to the continued killing that has cost nearly one quarter-million lives since 2011.

“Syria could be a battleground for a long time to come,” said Landis. The emergence of a hardline Islamist dictatorship led by al-Nusra, or even the Islamic State, or ISIS, would at least halt the rate of deaths seen during the past five years. “That’s an argument for why you need a victor, even a bad one.”

It’s an argument that Turkey and Saudi Arabia appear to be making, with or without the United States.