The Obama administration is publicly conflating the Islamic State (Isis) and al-Qaida, taking a legally convenient position for its new war that dismisses a major public split between the two jihadist organizations.

While several US officials contend the rupture between Isis and al-Qaida is irrelevant – Secretary of State John Kerry has mocked it as a “publicity stunt” – the administration line undercuts its previous distinctions between al-Qaida’s core leadership, various affiliates and unrelated terrorist groups.

Amongst counter-terrorism veterans, the conflation is considered tendentious – and, to some, reminiscent of the Bush administration’s exaggerated linkages between Saddam Hussein and al-Qaida, part of the language that tried to sell the 2003 Iraq invasion.

While Isis began life as al-Qaida in Iraq, al-Qaida’s leadership ultimately renounced all ties and condemned the group in February 2014. It is believed to be the first time al-Qaida has declared itself “not responsible” for a former affiliate.

“We know from open sourcing that they are not part of al-Qaida,” said Nada Bakos, a former CIA analyst. “Zawahiri denounced them. Baghdadi has declared his caliphate separate. We have no reason to believe they are currently operating as part of al-Qaida,” she said, referring to the respective leaders of al-Qaida and Isis.

Glenn Carle, a former CIA official who supports taking action against Isis, said that while the US public may not need a catalogue of the differences between Isis and al-Qaida, “each of them is different, and they are not one group.”

Much of the administration’s conflation of Isis and al-Qaida has occurred in a legal context, part of its argument that Obama possesses authority to attack Isis in Syria ahead of a congressional vote. But the contention is starting to migrate beyond legal discussions.

This US air forces Central Command image shows US aircraft participating in strikes in Syria and Iraq. Photograph: HO/AFP/Getty Images

On Tuesday at the Pentagon, Rear Admiral John Kirby, the spokesman, indicated that the military was not interested in distinguishing Isis from Jabhat al-Nusra, al-Qaida’s preferred Syrian affiliate – two groups have at times fought each other.

“Isil, though they claim to be broken off from al-Qaida, they were born of al-Qaida, and al-Nusra is an offshoot of al-Qaida. So in our minds, from our military perspective, they are very much one and the same,” Kirby told reporters.

“They can claim they’ve got differences with this or that group all they want, but we very much view them as one and the same.”

On Sunday, deputy national security adviser Antony Blinken declined to mention Isis’s split from al-Qaida at all, describing it through its former incarnation as al-Qaida in Iraq and referencing a founding member whom the US killed eight years ago.

“Zarqawi, the founder of al-Qaida in Iraq was a colleague of Osama bin Laden before [9/11], very close to him. After 9/11, he formed al-Qaida in Iraq. They associated themselves with al-Qaida,” Blinken told Fox News.

Bakos recalled debunking that understanding of Abu Musab al-Zarqawi for Bush administration officials ahead of the 2003 Iraq invasion, as Zarqawi’s network had yet to join forces with al-Qaida.

“It’s the same distinction we had to make prior to the Iraq war: Zarqawi was not part of al-Qaida, and we had to raise that,” Bakos said.

Even after the invasion, when Zarqawi formally joined al-Qaida, al-Qaida in Iraq’s relationship with al-Qaida’s core leadership was “strained” and “tenuous”, said Brian Fishman, a counter-terrorism scholar at the New America Foundation. Famously, Ayman Zawahiri, now al-Qaida’s leader, wrote Zarqawi a 2005 letter worrying that his brutality against civilians damaged al-Qaida’s brand.

Al-Qaeda’s Ayman al-Zawahiri wrote an Iraq affiliate in 2005 to say that their brutality damaged the group’s brand. Photograph: HO/AFP/Getty Images

Adam Gadahn, the most prominent American to join core al-Qaida, offered a withering critique of Isis in a document recovered by US forces during the 2011 raid that killed Osama bin Laden. Gadahn argued that “the only solution facing al-Qaida” was to disassociate itself “immediately” from what was then called the Islamic State of Iraq over “repulsive” attacks on Iraqi Christians and even mosques.

“The relations between al-Qaida organization and [the state] have been practically cut off for a number of years. The decision to declare the State was taken without consultation from al-Qaida leadership,” Gadahn wrote, according to a translation by West Point’s Combating Terrorism Center.

In March 2014, a federal criminal complaint against a California man who attempted to join Isis, Nicholas Michael Teausant, found that the would-be Isis adherent derided al-Qaida, which he believed received aid from western nations.

“It didn’t make a whole lot of sense for me for fighting for a group that’s getting paid by the people they’re trying to fight,” Teausant allegedly told an undercover officer.

But shortly after the US began bombing Isis targets in Syria, a senior US official described Isis’ split with al-Qaida as merely “a disagreement with al-Qaida leadership” while arguing that Obama never lost the legal ability to attack Isis under the 2001 Authorization to Use Military Force (AUMF), which Congress passed to retaliate for 9/11.

“We don’t believe that Congress would have intended to remove the President’s authority to use force against this group simply because the group had a disagreement with al-Qaida leadership,” the official told reporters in a background briefing last week.

Legally, the split poses a novel challenge for the US’s 13-year-long struggle against al-Qaida. The AUMF, a legal wellspring, only covers al-Qaida and its allies in complicity for 9/11, though subsequent interpretations, eventually entrenched into defense funding bills, permit the targeting of its “associated forces.” As recently as last year, Obama pledged publicly not to “sign laws designed to expand this mandate further.”

Seemingly unanticipated is the circumstance the US confronts: a group formally cast out of al-Qaida, operating in at least one country where the US is not engaged in formal hostilities, becoming amongst the most formidable jihadist entities.

Secretary of State John Kerry has argued to senators that Isis and al-Qaida are equivalently legitimate targets of US strikes. Photograph: Jason Szenes/EPA

A debate has emerged online as to whether the administration’s conflation of Isis with al-Qaida has stretched already-broad AUMF authorities to their breaking point. On blogs like Lawfare and Just Security, legal scholars pore over the claim in its textual minutiae - though those typically giving the idea credence, like Harold Koh and Cass Sunstein, are former Obama administration officials.

“Isis is not only saying that it’s separate from al-Qaida, but it’s actively fighting against an al-Qaida affiliate in Jabhat al-Nusra. So then what does the definition of an ‘affiliate’ mean?” said Fishman. “There are similar contours if you look at ideology and goals, but that’s not how we frame this in the law.”

The conflation is also a departure for an administration that has drawn nuanced distinctions between what it terms “core al-Qaida” and unaffiliated jihadist groups. Some of those distinctions have become politically damaging, as with the 2012 attack on the Benghazi consulate by a group without ties to al-Qaida, or with Obama’s much-mocked “junior varsity” line to the New Yorker about overstating a threat from marginal terrorists.

“I think there is a distinction between the capacity and reach of a bin Laden and a network that is actively planning major terrorist plots against the homeland versus jihadists who are engaged in various local power struggles and disputes, often sectarian,” Obama told the magazine’s David Remnick in January.

Daveed Gartenstein-Ross of the Foundation for the Defense of Democracies said that while he expected low-level Isis fighters defect to al-Qaida as the US-led war advances, “organizationally, no, they’ve crossed the threshold,” making it “clear and unambiguous” that the two groups are distinct.

“The rebuke by Zawahiri is a significant fact. The declaration of a caliphate and al-Qaida’s rejection of it is significant. More significant is that Isis has assassinated a number of al-Qaida figures,” said Gartenstein-Ross.