ISIS-K is trying to undermine Afghanistan's Taliban regime, from inside and out. That's America's problem, too.

In late September, Afghanistan's new Taliban leaders ordered their commanders to conduct a full background check on all fighters. The sudden move was prompted by concerns that other extremist groups have infiltrated the Taliban's ranks.

Senior members of the former Afghan government and one top Taliban official currently in a security role say the leader of ISIS-Khorasan (ISIS-K), a notorious terror operative whose identity has remained shrouded in mystery for years, is believed to be among the infiltrators. The officials spoke to CBS News on the condition of anonymity.

Some Taliban foot soldiers are believed to have abandoned the group to join ISIS-K or al Qaeda, preferring an even more extreme — and brutal — interpretation of Islam, but these sources say the ISIS faction is working deliberately to undermine the Taliban's authority from both inside and outside the group. That carries risks not only for Afghanistan, but for the United States and its allies.