The Taliban Tuesday warned the leader of the Takfiri group, ISIL against waging a parallel insurgency in Afghanistan, after a string of defections and reported clashes with militants loyal to ISIL.

The Taliban Tuesday warned the leader of the Takfiri group, ISIL (so-called Islamic State in Iraq and Levant) against waging a parallel insurgency in Afghanistan, after a string of defections and reported clashes with militants loyal to ISIL.

The Takfiri group operating in Iraq and Syria, also known by its Arabic acronym Daesh, has never formally acknowledged having a presence in Afghanistan but fears are growing that the group is making inroads in the country.

In a letter addressed to ISIL leader Abu Bakr al-Baghdadi, the Taliban insisted that "jihad (holy war) against the Americans and their allies must be conducted under one flag and one leadership".

"The Islamic Emirate (Taliban) does not consider the multiplicity of jihadi ranks beneficial either for jihad or for Muslims," said the letter signed by the Taliban deputy leader Mullah Akhtar Mohammad Mansoor.

"Your decisions taken from a distance will result in (the ISIL) losing support of religious scholars, mujahideen... and in order to defend its achievements the Islamic Emirate will be forced to react," it added.

The letter, published on the Taliban website in Pashto, Urdu, Arabic and Dari, did not elaborate on its threat.

The Taliban have seen defections in recent months -- with some insurgents apparently adopting the ISIL flag to rebrand themselves as a more lethal force as NATO troops depart.

The two groups are believed to be arrayed against each other in Afghanistan's restive south, with clashes frequently reported.

Last week, local media reported pitched battles between the Taliban and supporters of ISIL in eastern Afghanistan, with casualties reported on both sides.

General John Campbell, the commander of NATO forces in the country, last month said the ISIL group was recruiting militants in Afghanistan but they were not yet operational.