ISTANBUL -- On the battlefield in northern Syria, moderate rebel groups who used to dominate the struggle against President Bashar Assad are falling one after the other. Some of them, previously backed by the U.S., are now dissolved into radical Islamist factions. But one, a more extremist rebel group, has gained power with the help of Turkey, say rebel leaders in Aleppo, Syria. Turkey is forging its own path to try and topple Assad by actively supporting Ahrar al Sham, a group with ties to al Qaeda that the U.S. has for years kept at arm’s length -- and that strategy is pitting Turkey against Washington, its biggest ally.

The group was formed in 2011, recruiting Salafist and Islamist men predominantly in the Idlib and Aleppo areas. While the regime captured most of its founders, Ahrar al Sham continued to thrive, experts said, thanks to the support of wealthy businessmen in the Persian Gulf region and Turkey. The group has in the past been linked to Jabhat al-Nusra, the al Qaeda offshoot in Syria, and is known for its extremist and anti-Western political values. But like Jabhat al-Nusra, it does not have a reputation for brutality: It does not behave like the Islamic State group. In fact, it’s opposed to it.

Turkey’s support for Ahrar al Sham is representative of a larger conflict between President Recep Tayyip Erdogan’s government and the U.S. that is now coming to what analysts are calling a tipping point. Since the breakout of the Syrian civil war in 2011, Turkey and the U.S. have agreed on only one issue: the need to remove Assad from power. But that calculus changed with the rise of the Islamic State group -- also known as ISIS or ISIL -- and the U.S. has since focused firstly on defeating the Sunni militant group.

The two governments' tactics for intervention in Syria now differ drastically. Turkey has consistently backed Sunni Islamist rebel groups like Ahrar al Sham, while the U.S. has sought to support moderate rebels. But the latter have all but disappeared, merging into the very groups that the U.S. had tried to avoid dealing with altogether. That has left Turkey in almost complete control of the northern Syrian battlefield, through its local proxies.

After leading the fight to defeat Assad’s forces in Idlib, Ahrar al Sham is beginning to implement governing systems in the north, where the regime’s presence is weak. It may soon find itself also in control of one of the main border crossings between Syria and Turkey.

Last week the rebel group announced that it had handed over the Bab al Hawa crossing to a “civil authority” that would man the passport and border control office on the Syrian side, with no militants involved. The Turkish side of the border crossing is still closed, but local media sources say the change of hands on the Syrian side is the first step in the eventual re-opening of the Turkish side.

Who Is In Charge?

Who exactly is in charge of that civil authority is not clear yet. Sources within the now-defunct northern rebel groups told International Business Times that the civilians manning the border are in fact paid by Ahrar al Sham. Other rebel fighters said the civilians working the border crossing were actually overseen by the internationally recognized opposition interim government, a coalition with which Ahrar al-Sham is not aligned with and that controls some areas in the north of Syria.

In January the U.S. gave $6 million to the interim opposition government. At the time of the grant, a senior rebel official told Agence France-Presse that the funds would be used mostly in northern Aleppo -- an area that includes the Bab al Hawa crossing. Turkey, though, has targeted its assistance directly to Ahrar al Sham, rebel fighters said. The Turkish prime minister’s office could not be reached for comment.

That Turkish support, analysts say, comes in the form of weapons and cash transported through border crossings like the one at Bab al Hawa. Keeping them under the control of Ahrar al Sham benefits Turkey.

Sine the beginning of the Syrian civil war, “the Turkish government had a policy of perhaps not actively supporting, but looking the other way, when people crossed the border to support the various factions fighting the Assad regime,” said Marc Pierini, the former European Union ambassador to Turkey as well as to Syria. “Some people claimed there were arms delivery and active support from Turkey.”

In a Foreign Affairs magazine interview in January, Assad said that Erdogan was “personally responsible” for the fighting in Syria and the surge of Islamic terrorism in the region. “There will be no problem defeating [ISIS]. Even today we don’t have a problem militarily. The problem is that they still have this continuous supply, mainly from Turkey,” Assad said.

But whatever Turkish help Ahrar al Sham may have received, the support of the United States, the leader of the coalition fighting ISIS, remains crucial. And so far, the U.S. has not endorsed Ahrar al Sham, or any other extremist group in the north of Syria. It first wants Turkey to do more to control the flow of foreign fighters entering Syria to fight with ISIS.

“Turkey is not cooperating fully with the anti-ISIS coalition,” Pierini, now a visiting scholar at Carnegie Europe, said. “European governments have approached the Turkish government and asked that a serious effort be made not to let the jihadists cross into Syria. Some progress has been made, but nevertheless what you have now is more or less the border staying quite open. You can still see people are moving back and forth.”

From the outside, though, it looks like Turkey has been making an effort, especially in the last few weeks, to secure its borders. It has arrested dozens of ISIS suspects and has closed its borders into Syria, except for relief trucks. And letting a civil authority control the Bab al Hawa crossing may be a move by Turkey “to mitigate the concerns that the Western community has about Ahrar al-Sham … playing a prominent role in the northern [Syria] offensive,” said Andrew Tabler, an expert on Syria at the Washington Institute For Near East Policy.

'Simply Puzzled'

Washington has been without a credible horse to back in the region for months. The last U.S.-backed rebel faction, Harakat Hazzm, disbanded earlier this year. So far, analysts say, the U.S. does not have a plan for supporting any given group in northern Syria, though it has started to vet groups in the south to arm. The U.S. and the coalition still need reassurance from Turkey, the most influential and militarily strongest power in the region, to assuage their fears that any weapons they provide may eventually land in the hands of extremist groups.

“A lot of Western observers are simply puzzled by the way Turkey has handled this subject,” Pierini said. “On the one hand Turkey says it is against violence and that ISIS is a terrorist organization. But at the same time the anti-Western narrative is something that is regarded with sympathy by the religious conservatives in Turkey and the electorate.”

Turkish general elections are set for June 7. Erdogan is looking for a sweeping victory that will give his AKP party the ability to change the constitution and broaden his presidential powers. [AKP is the Turkish abbreviation for Justice and Development Party.] He has historically appealed to the conservative Muslim population in Turkey, most of whom opposed the country’s joining of the U.S.-led coalition last year.

In an effort to keep those voters on his side, even though he is not officially able to campaign, Erdogan is expected to continue to appeal to the conservative Muslim narrative and to ramp up his anti-American rhetoric, as he has done in the past.

That push by Erdogan may lead to more divisions between the U.S. and Turkey, and could help prolong the stalemate on the ground in Syria -- where ISIS controls the east, the regime holds the west and south, and rebels hold the north, as the war enters its fifth year.