A deal between the Taliban and the United States for US forces to withdraw from their longest-ever war in Afghanistan could drive some diehard Taliban fighters into the arms of the Islamic State militant group, Afghan officials and militants say.

Such a deal is expected to see the United States agree to withdraw its forces in exchange for a Taliban promise they will not let Afghanistan be used to plot international militant attacks.

As part of the pact, the Taliban are expected to make a commitment to power-sharing talks with the US-backed government and work out a ceasefire.

The Afghan affiliate of Islamic State, known as Islamic State Khorasan (ISIS-K), after an old name for the region, first appeared in eastern Afghanistan in 2014, and has since made inroads into other areas, particularly the north.

The US military estimates their strength at 2,000 fighters. Some Afghan officials estimated the number is higher, and could be about to get a boost.

“It’s a big opportunity for Daesh to recruit fighters from the Taliban, and, no doubt, many Taliban fighters will happily join,” said Sohrab Qaderi, a member of the provincial council in Nangarhar province on the border with Pakistan, referring to IS.

For some Taliban, IS will offer an opportunity to continue jihad against those they see as infidels and their supporters. For others, who fear retribution if they try to reintegrate into society, it could be a refuge.

“They’ve killed and been killed, they have feuds,” Qaderi said of the Taliban. “Many fighters won’t feel safe returning to normal life.”