HALIFAX - Leaders from more than 50 countries are gathering at a downtown hotel in Halifax for a global security conference, but one high-profile security official is noticeably absent.

Former CIA director David Petraeus was among those expected to speak at the International Security Forum.

Petraeus cancelled after resigning his post once an extra-marital affair became public.

Defence Minister Peter MacKay is expected to open the conference.

Delegates will be discussing pressing security issues, including the impact of the American presidential election, the turmoil in Syria, cybersecurity and modern warfare.