Rohan Gunaratna is a Professor of Security Studies at the S. Rajaratnam School of International Studies, Nanyang Technology University, and head of the International Centre for Political Violence and Terrorism Research, Singapore. He is the author of 16 books including "Inside al Qaeda: Global Network of Terror." The opinions expressed here are solely his.

(CNN) With the ISIS heartland in Iraq and Syria imploding, its support groups in the periphery, as well as rival groups such as al Qaeda, are gathering momentum.

With the likely dwindling of ISIS power, will there be a reverse defection where the groups that previously left al Qaeda rejoin the movement? Or, will the so-called Islamic State reinvent and refocus on weakly governed sub-regions and countries?

The epicenter of international terrorism started to shift from Afghanistan to Iraq after US invasion of Iraq in March 2003 and reached its peak after the Syrian uprising in 2011.

But, as it suffers in its heartland, it is very likely that ISIS will grow in size, strength and influence in Afghanistan and Pakistan.

Introducing ISK

ISIS has lured several Pakistani, Afghan and foreign groups since June 2014.

Gradually, ISIS united these diverse entities, creating Islamic State Khorasan (ISK) in January 2015.

Lashkar-e-Jhangvi, the group most recently blamed for the police academy attack in Quetta, split after the Pakistani authorities decapitated its leadership

Its Karachi faction is now working with AQIS -- al Qaeda in the Sub Continent, while its Baluchistan faction is working with ISK.

JUST WATCHED Pakistan hospital bombing targets lawyers Replay More Videos ... MUST WATCH Pakistan hospital bombing targets lawyers 02:32

As such, the attacks mounted are claimed by both ISIS and the perpetrating groups.

Starting in April 2015, ISK has staged two dozen attacks, mostly suicide attacks in Afghanistan and Pakistan, killing over 200 civilians and security forces personnel.

Most recently, on Monday, ISK attacked the Police Training College in Quetta in Western Pakistan, killing 61 and injuring 117. The cadets were unarmed and sleeping when three militants entered through the window of the barracks carrying suicide bombs.

Other attacks demonstrate that the threat posed by ISIS has spread from tribal to mainland Pakistan. These include a May 2015 strike, where ISK gunmen attacked a bus carrying members of a religious minority killing 45 and injuring dozens as well as an August 2016 attack, in which 72 were killed in an attack on a murdered lawyer's funeral.

Afghan attacks slammed by Taliban

The group's activity has also intensified in Afghanistan, where ISK is targeting not only security and coalition forces, but civilians.

JUST WATCHED ISIS claims responsibility for deadly Kabul blasts Replay More Videos ... MUST WATCH ISIS claims responsibility for deadly Kabul blasts 03:26

In October, ISK claimed the killing of Staff Sgt. Adam S. Thomas , an American soldier in Achin district of Nangarhar. In the same month, the group also mounted a series of attacks against Shi'ite shrines.

At least 14 people were killed at the Karte Sakhi shrine in the Afghan capital, Kabul, during religious observations for the Day of Ashura, a holy day of mourning.

The Afghan Taliban slammed the shrine attacks, saying "they represent an attack on all Afghans, and a means by the enemy to sow sectarian hatred and violence."

The Afghan Taliban did not mention ISK and its claim for the operations in its statement; rather, it said "no matter who perpetuated them and under what excuse".

"Once the flames of communal violence are ignited, the Afghans will cannibalize each other, weaken their unity, actively seek foreign assistance, and ultimately welcome any foreign intervention that helps in repressing the opposition forces," the group said in a statement.

JUST WATCHED ISIS claims responsibility for suicide attack Replay More Videos ... MUST WATCH ISIS claims responsibility for suicide attack 02:33

"Such an eventuality will only prolong the current or any future occupation of our nation, divide us politically, ethnically, militarily and ultimately facilitate the dismemberment of our beloved country."

Other attacks claimed by ISK in Afghanistan include a suicide bomber on a motorbike who killed at least 33 outside a bank in Jalalabad in April 2015, a joint suicide bombing that killed 80 Hazaras during a peaceful demonstration in July of this year and the execution of eight Taliban members in Nangarhar in August.

Increasingly, the tactics used by ISK mirror the beheadings and other forms of killing perpetrated by ISIS in its heartland of Iraq and Syria.

'The Af-Pak theater'

ISIS is transforming from a pretentious caliphate-building group to a global terrorist movement, and of all the poorly governed regions, the Af-Pak theater is most ripe for the rise of the ISK.

Afghanistan Taliban, Pakistani Taliban, Al Qaeda in the Indian Sub Continent (AQIS) and another dozen groups challenge ISK, but the appeal of ISIS to a tiny segment of the South Asian population is undeniable.

As a part of its decentralization strategy, it's clear ISIS has seeded a nucleus in the Af-Pak theater. And despite military reversals in Iraq and Syria, it is very likely ISIS will grow in size, strength and influence in Afghanistan and Pakistan.

ISIS has suffered in its heartland, but its appeal remains significant, and the sheer brutality of ISK will sustain the momentum and spread in parts of Pakistan and Afghanistan.

Unless the Afghanistan and Pakistan government work together, the threat is likely to grow and affect the entire South Asian region. Already, ISK has established cells in India and ISIS has established a presence in Sri Lanka and Maldives.

ISIS presents a growing threat to the entire South Asian region. With a Muslim population of 240 million or 40% of the global Muslim population -- South Asia is a lucrative battlefield for ISIS.

Governments within and outside the region should work with Pakistan and Afghanistan to contain, isolate and eliminate ISK and the influence of ISIS.