In a joint statement released Tuesday, two powerful al-Qaeda affiliates urged Muslims to unite against the U.S.-led coalition targeting the Islamic State.

In a two-page message posted to Twitter accounts that represent both groups, al-Qaeda in the Arabian Peninsula (AQAP) and al-Qaeda in the Islamic Maghreb (AQIM) asked their "brothers" in Iraq and Syria to "stop killing each other and unite against the American campaign and its evil coalition that threatens us all."

It's an unusual move. The two groups are perhaps the most notorious of the al-Qaeda-linked groups: AQAP operates in Yemen and Saudi Arabia, and it has been described as the "most lethal Qaeda franchise" by the Council on Foreign Relations, while AQIM operates in Northern Africa, in particular Algeria, Mali and Libya. Analysts say a joint statement from the two is unprecedented.

If the message were to be interpreted as a sign of support for the Islamic State, it's more surprising still: Both groups formally declare allegiance to al-Qaeda's central leadership, led by Ayman al-Zawahiri after the death of Osama bin Laden. And while the Islamic State may have once been thought of as an al-Qaeda offshoot, Zawahiri's central command has disowned it, and relations appear fraught: In February, al-Qaeda released a statement that said the Islamic State “is not a branch of the al-Qaeda group," adding that al-Qaeda does not "have an organizational relationship with it and [al-Qaeda] is not the group responsible for their actions." In the Syrian civil war, al-Qaeda backs its own proxy, Jabhat al-Nusra, widely seen as less extreme than the Islamic State. The group also has links to Ahrar al-Sham, the largest militia in the Islamic Front coalition battling the government of President Bashar al-Assad. Both Jabhat al-Nusra and Ahrar al-Sham are opposed to the Islamic State, and there is some speculation that the Islamic State could have been behind a mysterious explosion that killed much of the Ahrar al-Sham leadership recently.

While the Islamic State was once linked to al-Qaeda (which, in turn, forms a good chunk of the U.S. legal basis for attacking it), it is more accurate to now think of the two groups as rivals. In a recent paper for the Washington Institute for Near East Policy, Aaron Y. Zelin outlined some of the key issues on which the groups differ, including the use of violence, the role of Islamic government and the strategic value of alliances. "The two groups are now in an open war for supremacy of the global jihadist movement," Zelin wrote.

One place that this "war" will be played out is among the affiliates: As my colleague Ishaan Tharoor has pointed out, the announcement of a new al-Qaeda wing in South Asia seems to be prompted as much by the terror network's rivalry with the Islamic State as anything else. AQAP's leadership has offered cautious messages of support for the Islamic State before, and this week a group of AQIM commanders broke away from their leadership to pledge allegiance to the Islamic State. The appeal of the Islamic State is obvious: Right now, the group clearly has a momentum that al-Qaeda lacks. Its dramatic gains in Syria and Iraq have led to vast infusions of cash and equipment, and its violent tactics (which al-Qaeda condemns) seem to be a potent recruiting tool.

Still, the joint statement released Wednesday is not quite an unequivocal document of support for the Islamic State. Its wording is careful, and notably it includes a message of condolence for the Ahrar al-Sham leadership. As terrorism analyst J.M. Berger tweeted, the groups appeared to be "treading carefully around the Islamic State, playing the middle."

Instead, the statement calls on all jihadist groups to unite against a common enemy: "crusader America" and the alliance of states backing the U.S. plan to strike the Islamic State. This language echoes the Islamic State's own language and presents a bigger concern: Might U.S. strikes against the Islamic State cause it to reunite with al-Qaeda and other extremist groups it opposes?