"If [the Islamic State] becomes much more of a factor in the [Afghan] insurgency, it will connect the Middle East and South Asia," Nasr warned during an Atlantic Council discussion on the future of Afghanistan.
The spread of the Islamic State from its current strongholds in the Middle East and North Africa would catch the United States off guard, Nasr noted.
"We do not have right now, if you were to say, a single strategy that fits both the Middle East and South Asia," Nasr said.
There are as many as 4,000 Islamic State militants in Afghanistan, according to estimates from the Russian General Staff.
Since the US withdrawal of forces in December 2014, Afghanistan has seen a revived Taliban insurgency as well as other security threats.
On Thursday, President Barack Obama announced the United States would keep a troop presence of 9,800 in Afghanistan, drawing those levels down to 5,500 by 2016 or 2017.
