The US intelligence community has assessed that the Islamic State in Afghanistan could have the capability to attack the US in as little as six months, and has the intention to do so, a senior Pentagon official told Congress on Tuesday.

The remarks by Colin Kahl, under secretary of defence for policy, are the latest reminder that Afghanistan could still pose serious national security concerns for the US even after it ended its two-decade-old war in defeat in August.

The Taliban, which won the war, are enemies of the IS and have seen its attempts to impose law and order after the US pullout thwarted by suicide bombings and other attacks claimed by the IS.

They include bombings targeting the minority Shia sect and even an IS beheading of a member of a Taliban militia force in Jalalabad.

In testimony before the Senate Armed Services Committee, Kahl said it was still unclear whether the Taliban has the ability to fight the IS effectively following the US withdrawal in August.