The top U.S. commander in Afghanistan, General John Nicholson, said U.S. drone strikes and special forces operations had killed about a third of IS fighters in Afghanistan and cut their territory by two thirds.
U.S. officials say intelligence suggests IS is based overwhelmingly in Nangarhar and neighboring Kunar province. They are skeptical about reports of an increased IS presence in the northwest, where gunmen may claim a connection to the group to boost their standing.
"Certainly if you're a local official who's looking for more resources, by saying that ISIS is in your area, you're going to get more attention," said Brigadier General Charles Cleveland, the main U.S. military spokesman in Kabul.
Nicholson, who has asked for thousands more troops in Afghanistan, said counter-terrorism forces planned a series of operations in 2017 to defeat IS in Afghanistan "and preclude the migration of terrorists from Iraq and Syria into Afghanistan".
Still, bolstered by fighters from Pakistani and other militant groups and an active social media presence, some Afghan officials said it had become an "umbrella organization" for disparate movements.
The Afghan Taliban, waging war to overthrow the U.S.-backed government in Kabul, are fiercely opposed to IS, and the two have clashed as they seek to expand territory and influence.
"We have almost eliminated Daesh in Afghanistan," said Taliban spokesman Zabihullah Mujahid.
Estimating numbers is difficult. Cleveland said U.S. officials believe the movement has only 700 fighters, but Afghan officials estimate it has around 1,500, with twice as many auxiliary helpers and up to 8,000 less active supporters.
Those officials say fighters from Central Asian countries including Uzbekistan have come to Afghanistan, while militants forced across the border by Pakistani military operations also gravitated toward IS.
Khorasani, the IS militant, said it had "thousands" of fighters and was growing.
A U.S. drone strike last year killed former local IS leader Hafiz Saeed Khan, once a member of the Pakistani Taliban. Several Afghan security officials believe a former Afghan Taliban commander, Abdul Haseeb Logari, has replaced him.
Islamic State began to be noticed in the region in early 2015, when loyalists took on the Taliban and al Qaeda in a bid to become the leading Islamist militant group, using tactics that stood out for their brutality even in Afghanistan.