BERLIN (AP) — NATO's secretary general says the death of Islamic State leader Abu Bakr al-Baghdadi is a "milestone" in the fight against the extremist group but is cautioning that the struggle is far from over.

NATO Jens Stoltenberg told Germany's Bild newspaper on Sunday that IS once controlled a vast area in Syria and Iraq and the international anti-IS coalition, of which NATO is a part, needs to "ensure that it doesn't return."

Stoltenberg says "the Islamic State doesn't have any territory any more, but it still lives. IS maintains sleeper cells, secret networks and is working to come back. Our mission is not yet entirely fulfilled."

Al-Baghdadi was killed last week in a U.S. raid in Syria. The group has named Abu Ibrahim al-Hashemi Al-Qurayshi as its new leader.