WASHINGTON - The Islamic State in Iraq and Syria (ISIS) group is in an "initial exploratory phase" in Afghanistan, the Pentagon said in a report to Congress, noting the Taleban retained its resilience in the war-torn country.

ISIS extremists have not formally acknowledged having a presence in Afghanistan, but fears are growing that the group is making inroads there.

The Afghan government and the US-led international coalition fighting the Taleban are keeping a wary eye on ISIS, which has grabbed large areas of Syria and Iraq in a brutal offensive involving beheadings and forced religious conversions.

"All are collaborating closely in order to prevent this threat from expanding," the Pentagon report said, citing "evidence of limited recruiting efforts" by ISIS in Afghanistan. "Yet ISIS' presence and influence in Afghanistan remain in the initial exploratory phase."

On Tuesday, the Taleban warned the leader of ISIS against waging a parallel insurgency in Afghanistan, after a string of defections and reported clashes with militants loyal to ISIS.

"There is no room for a parallel Islamic army in Afghanistan," Afghan Taleban deputy leader Mullah Akhtar Mohammad Mansour wrote to ISIS leader Abu Bakr al-Baghdadi. "Jihad against American occupiers and their Afghan mercenaries must be held under one flag, one leadership and one command."

The Pentagon report said that while "ISIS will likely continue to try to expand its presence in Afghanistan during the upcoming year", the "resilient" Taleban-led insurgency is still the major threat to the US, coalition and Afghan security forces.

AGENCE FRANCE-PRESSE, BLOOMBERG