The collapse of the Islamic State's self-declared caliphate in Syria and Iraq is doing little to slow down the terror group's branch in Afghanistan.

Newly unclassified intelligence suggests IS-Khorasan, as the group is known, is growing both in numbers and ambition, now boasting as many as 5,000 fighters — nearly five times as many as estimates from last year — while turning its focus to bigger and more spectacular attacks.

Military officials say the numbers, shared by U.S. Forces-Afghanistan for the latest quarterly report by the Lead Inspector General for Operation Freedom Sentinel, issued Tuesday, are "low confidence" estimates but that IS-Khorasan has fighters in Kabul, as well as in Nangarhar and Kunar provinces and in other parts of northeastern Afghanistan.

More worrisome, according to defense intelligence officials, is that the terror group has been gaining ground, both against the Afghan government and the Taliban, expanding the amount of territory under its control in Kunar province since the start of the year.

It also appears IS-Khorasan has managed to gain a toe-hold in other areas. The assessment finds it is "highly likely" IS has smaller cells operating in parts of Afghanistan under either government or Taliban control.

"We're very concerned about their capability and trajectory," Col. Dave Butler, the spokesman for U.S. Forces-Afghanistan, told VOA. "IS-K has made it clear that they aspire to attack the United States and our allies."

Afghan Special Forces inspect inside a cave which FILE - Afghan Special Forces inspect a cave which was used by suspected Islamic State militants at the site where a MOAB, or ''mother of all bombs'', struck the Achin district of the eastern province of Nangarhar, Afghanistan, April 23, 2017. FILE - Afghan Special Forces inspect a cave which was used by suspected Islamic State militants at the site where a MOAB, or ''mother of all bombs'', struck the Achin district of the eastern province of Nangarhar, Afghanistan, April 23, 2017.

?The current state of IS in Afghanistan would appear to represent a remarkable turnaround from the terror group's fortunes in April 2017, when the U.S. dropped the largest non-nuclear bomb in the U.S. arsenal, a GBU-43 Massive Ordnance Air Blast, on an IS cave and tunnel system in Nangarhar province.

A series of subsequent U.S. strikes killed the then IS-Khorasan emir and his replacement, and cut the estimated number of fighters from 3,000 to 600.

IS in Afghanistan Just Won't Go Away, US Officials Say Intensified efforts to root out and destroy the Islamic State terror group in Afghanistan are making progress in some areas but have so far failed to prevent the terror group from maintaining a foothold in the country, based on the latest U.S. Intensified efforts to root out and destroy the Islamic State terror group in Afghanistan are making progress in some areas but have so far failed to prevent the terror group from maintaining a foothold in the country, based on the latest U.S.

As recently as late last year, top U.S. military officials insisted IS-Khorasan was being kept in check.

"ISIS-K is not growing," Gen. John Nicholson, the outgoing commander of U.S. Forces-Afghanistan, said in September before handing over the command to Gen. Scott Miller.

Islamic State 'Not Growing' in Afghanistan, Nicholson Says The Islamic State terror group is losing its grip on parts of Afghanistan, slowly succumbing to pressure from U.S. and Afghan forces, the outgoing commander of U.S. Forces-Afghanistan said Saturday. General John Nicholson rejected the notion that IS-Khorsasan, also known as ISIS-K or IS-K, has been able to meaningfully expand its presence following a concerted effort to wipe it out, which began last year. That effort included use of the largest non-nuclear bomb in the U.S. arsenal. "ISIS-K is not… The Islamic State terror group is losing its grip on parts of Afghanistan, slowly succumbing to pressure from U.S. and Afghan forces, the outgoing commander of U.S. Forces-Afghanistan said Saturday. General John Nicholson rejected the notion that IS-Khorsasan, also known as ISIS-K or IS-K, has been able to meaningfully expand its presence following a concerted effort to wipe it out, which began last year. That effort included use of the largest non-nuclear bomb in the U.S. arsenal. "ISIS-K is not…

Yet despite losing key leaders and suffering constant setbacks on the battlefield, IS-Khorasan found a way to maintain its numbers, successfully recruiting disgruntled Taliban fighters as well as jihadis from further afield.

Afghan officials have likewise warned that IS-Khorasan has benefited from surges of thousands of foreign fighters from Pakistan and Uzbekistan, plus the inflow of hundreds of IS fighters fleeing from Syria.

Even in Defeat, Islamic State's Foreign Fighters Expanding This is part two of a four-part series.WASHINGTON — It was March 2018, approximately five months after U.S.-backed forces had declared victory in the battle for the Islamic State terror group's Syrian capital of Raqqa. And Afghan officials had reason to worry.Intelligence suggested the fallout from that victory had been making its way, slowly but surely, to their country."There has been a growth in the number of the foreign fighters in the country," then-Afghan national security adviser Mohammad Hanif… This is part two of a four-part series.WASHINGTON — It was March 2018, approximately five months after U.S.-backed forces had declared victory in the battle for the Islamic State terror group's Syrian capital of Raqqa. And Afghan officials had reason to worry.Intelligence suggested the fallout from that victory had been making its way, slowly but surely, to their country."There has been a growth in the number of the foreign fighters in the country," then-Afghan national security adviser Mohammad Hanif…

But increasingly, many of IS-Khorasan's recruits are local, with fewer coming from the ranks of disgruntled Taliban fighters. And the group has even taken a page out of IS core's playbook, targeting young men seeking economic opportunity.

"IS does a lot of recruiting in Afghanistan and the region attempting to enlist disaffected youth and even tech-savvy young people looking for an alternative," Butler said.

An Afghan soldier keeps watch during clashes with FILE - An Afghan soldier keeps watch during clashes with Islamic State militants in Kot district of Nangarhar province, Afghanistan, June 26, 2016. FILE - An Afghan soldier keeps watch during clashes with Islamic State militants in Kot district of Nangarhar province, Afghanistan, June 26, 2016.

The U.S. and Afghanistan are not the only countries voicing concern about the growing IS-Khorasan presence.

Russia's top intelligence official, FSB chief Alexander Bortnikov, on Tuesday warned that 5,000 IS foreign fighters, many of whom have fought in Syria, were massing along Afghanistan's northern border, potentially threatening former Soviet states in the region.

Bortnikov made the comments while visiting Tajikistan, which has blamed a deadly May 19 riot at one of its high-security prisons on IS followers who were being held there.

Many Western intelligence officials and analysts have long been skeptical of Russia's claims when it comes to IS in Afghanistan, noting Moscow is prone to inflate numbers to serve its own interests in the region.