AMMAN - Tensions are high along the Syria-Jordan border, after a conflict between opposition groups almost saw their alliance disintegrate in the midst of a ruthless new government offensive.

Dera’a province in southern Syria has long been a Free Syrian Army (FSA) stronghold. It’s where the uprising began in the spring of 2011, and it’s home to the al-Omari Brigade and the Yarmouk Division, some of the moderate FSA’s most powerful factions.

Yet Dera’a is also home to Islamists who fight with the al-Qaeda franchise Jabhat al-Nusra. Over the past few weeks, tensions between these ideologically divergent groups have risen sharply.

“The FSA and Nusra are fighting against the same enemy,” said an activist from the Tahrir Souri media network, who operates anonymously for his own and his family’s protection. “But the relationship between the two groups is undefined, and it is being tested in Dera’a.”

For much of the conflict, local FSA factions have fought alongside better-funded and better-equipped Nusra fighters. While Nusra is known to include many foreign fighters in its ranks, most of those fighting in Dera'a are locals, often friends and kin of FSA fighters. FSA spokesman Abu Omar Al-Hourani puts the number of local fighters on the Nusra side at 80 percent.

Two weeks ago, a major rift between these opposition groups was exposed when members of Nusra arrested a group of men, including FSA Colonel Ahmad al-Ni’ma. The charge: treason - specifically, forfeiting a strategically vital spring 2013 battle to the regime at the request of Western backers.

Nusra wasn’t alone in alleging that Ni’ma had acted questionably. Several high-profile FSA commanders had called for an investigation into the battle, which was lost almost inexplicably late in the game after a series of disastrous leadership calls. But the arrest was made by Nusra alone, which suggests an executive decision to take on a policeman-type role in the region.

Predictably, this didn’t sit well with the FSA. Shortly after the arrest, a response was posted online by several FSA factions, including the Yarmouk Division - one of nine factions deemed moderate and stable enough to receive American support in the form of TOW anti-tank missiles. The statement demanded the men’s release and called a halt on collaboration between the FSA and Nusra. It pre-emptively disavowed any confessions obtained, and charged Nusra with ensuring the men’s safety.

But within hours of being posted, the message disappeared: some of the 59 separate FSA factions listed as signatories did not agree with it.

The arrest also had reverberations in Jordan. Although the border in Dera’a has been closed to refugees for the better part of a year, it has quietly remained open to a trickle of vetted FSA fighters and other approved traffic, including ambulances, according to organisations and individuals who have used the border regularly since the conflict began. The day after the arrest, the border slammed shut - almost - with only the most grievously injured people still allowed in, the same parties said.

This is indicative of the enormous wariness with which Jordanian intelligence regards the extremist group, said a Damascus-based government source, speaking anonymously as he is not authorised to comment on behalf of the regime. “Jordan is shutting down on Nusra, not the FSA.”

Craig Larkin, a political scientist at Kings College London who specialises in the region, agreed. “The Dera’a front is becoming more significant, and Jordanian intelligence is getting very worried about an escalation here.”

Over the following days, newspapers and activists reported that negotiations took place between the FSA, Nusra and, allegedly, the FSA’s Jordanian backers. These dealings resulted in the release of several men, but Ni’ma, the highest-profile detainee, remained in custody. “Dera’a was on a knife edge”, said the Tahrir Souri activist.

An anticipated confessional video was released, featuring a bruised Ni’ma admitting to colluding with international backers in the defeat at Khirbet Ghazaleh, the battle in question.

More than just a shaming of Ni’ma, Larkin said, this was a deliberate dig at the whole organisation. “Nusra’s use of the confession was an indictment of the FSA, that they were stooges of the West. That this happened when the Syrian Military Council was reaching out to the US for greater support doesn’t help.”

The rift tore even deeper when, days later, about 80 men and boys demonstrated against Nusra in the Dera’a town of Ghariyeh - a town where the group does not maintain a formal presence. Placards demanded Nusra “Release the detainees, you have lost your public support. We are all Ahmad al-Ni’ma.” Another obliquely referenced Nusra’s foreign fighters: “You are not going to rule us in our own country.”

But the demonstration was a one-off and no one was released. Muhamed Shukri, a pro-FSA activist reached in his hometown of Ghariyeh, admitted the protest told only a small part of the story, and that support was shifting away from the FSA: “Most of the young people from my town are in favour of Nusra.”

Then at dawn last Thursday, the regime launched a powerful new offensive, with three battles opening up: in the city of Dera’a, in the town of Nawa in the north of the province, and in the town of Inkhel, in western Dera’a.

Under heavy bombardment all weekend, the FSA and Nusra fought their common enemy as they had so many times before. But the nature of the fight seems to be changing.

“Nusra and the FSA are not collaborating in any way, shape or form,” said the Tahrir Souri activist. “They are fighting the same enemy, but where before there was an operations room shared by Nusra and the FSA, now there is an FSA operations room and a Nusra operations room.”

In Dera’a, pro-FSA and pro-Nusra activists spoke of a relationship under strain, but denied a full-on split in the opposition.

“There hasn’t been actual fighting between the FSA and Nusra, only tension,” said Omar al-Hariri, a Dera’a journalist with activist-based Sham News Network. “And not all factions of the FSA have a problem with Nusra. It’s just a few.”

Mohammed Ktefan, a Nusra supporter from Dera’a, agreed the two groups were still working together: “The FSA and Nusra have a lot in common and they are fighting the regime together on a number of fronts.”

Beyond the talk of battlefield collaboration, Ktefan described a shift in loyalties towards his organisation. “A large number of FSA factions support the arrest of Ahmad al-Ni’ma. He didn’t just hurt the Islamic factions, but all the people of Dera’a. His arrest gave people a positive feeling, not the opposite.”

This change in loyalties may be a bellwether for a much more significant shift, said Larkin. “Nusra have been very active coordinating military actions in Dera’a, and a lot of people have been dissatisfied with the FSA for a long time.”

“There’s a battle here to see who represents moderate Islamists, and better-organised, more strategic Nusra has been winning hearts and minds. We’re seeing a polarisation of rebel forces in the south.”

But in the past 48 hours, the opposition landscape has shifted on a national scale, as Islamic rebel groups including the Islamic Front signed a revolutionary covenant that forbids fundamentalism and radicalism.

Radical, al-Qaeda-aligned Nusra isn’t included here. And this raises the possibility that the group might not survive: the most radical fighters could join a more radical group, and more moderate fighters could fall in with the Islamic Front.

From the perspective of the Syrian regime, any escalation of tensions in opposition ranks is a good thing, said the source in Damascus. “There are vast ideological differences between Nusra and the FSA. The government expected infighting, and we expect more to come.”

And for now, at least, it continues to come. At the start of the third week of Ni’ma’s detention, three separate, heavyweight battles continued to tear Dera’a apart.

Jordan is keeping its border closed, wary of jihadist elements so close to home, while there are reports that the US is considering handing heavier weapons to those it could trust.

The Tahrir Souri activist said those in the know about evolving weapons talks with the Americans describe any potential arrangement as “if you fight our enemy, we’ll give you weapons to fight yours.”

The condition, said the source in Damascus, is that the FSA must be free of any links to al-Qaeda. And the activist says this won’t be so easy. “While many people within the FSA say they’ll fight al-Qaeda, they’re not so eager to fight Nusra. It hasn’t really wronged them and they need its support.”

Nowhere is this more true than in Dera’a, where at the time of press, FSA and Nusra fighters were firing from shared frontlines against a regime offensive.

For now, the uneasy alliance seems to be holding.