Self-proclaimed Islamic State militants are gaining a foothold and conducting limited military operations in Afghanistan, for now primarily targeting Taliban insurgents and their territory, a defense official said.

The apparent growth of the Islamic State group in Afghanistan is a "great concern" to U.S commanders and could influence plans for ending the 14-year American military mission there next year, the defense official said.

"It's a problem because it's a destabilizing influence," said Army Brig. Gen. Wilson Shoffner, deputy chief of staff for communications for the mission, known as the Resolute Support.

"It's clearly a factor that will be included — one of many factors that will be included as part of the [commanders'] assessment at the end of this year," Shoffner said.

As Afghanistan's fighting season ends this fall, the top U.S. commander, Army Gen. John Campbell, will review the plans to withdraw the roughly 10,000 American troops by the end of 2016.

"We categorize 'Daesh' in Afghanistan as 'operationally emergent,' " Shoffner told reporters in a briefing from Afghanistan, referring to Islamic State militants by an Arabic acronym.

For now, military commanders do not consider the Islamic State group to have "operational capability" because "we do not see them having the ability to coordinate operations in more than one part of the country at a time," Shoffner said.

"We do have reports of them operating in different parts the county. But ... not in a coordinated fashion," he said.

"We see their capability increasing somewhat but not to the point that they can conduct the kind of operations you're seeing in Iraq and Syria," Shoffner said.

"We do note the potential for them to evolve into something more serious, more dangerous. We take that very seriously," he said.

"The fighting has generally been Daesh against the Taliban," Shoffner said.

"Surely this is the result of Daesh encroaching on Taliban territory and interfering with established Taliban operations," he said.

The most intense fighting between the Islamic State and the Taliban has been in the Taliban stronghold of Nangarhar province, near Jalalabad, he said.

"Most of what we're seeing is attempts to gain influence ... recruiting. Attempts to gain territory and an attempt to disrupt the Taliban in terms of their operation," he said.

"We do see some funding flowing to Daesh but not a significant amount," he said.

It's unclear whether the self-proclaimed Islamic State fighters in Afghanistan have operational links or a command-and-control relationship with the Islamic State's central organization in Iraq and Syria.

"We have seen some Taliban rebrand themselves as Daesh. We're not exactly sure why this is. We believe it's probably an attempt to gain resources or perhaps attention or better leadership," Shoffner said.

In July, the Afghan intelligence agency announced that U.S. aircraft killed a regional Islamic State leader in Nangahar province.

U.S. and coalition aircraft have dropped about 400 bombs on Afghanistan this year, but Shoffner declined to say the extent to which those strikes are targeting Islamic State militants.

"The commander here has the authority he needs. We conduct drone strikes for two reasons: that's counterterrorism and force protection. And I won't go into any details on specific targeting."