The US-based tracker of extremist activity published an image, purportedly of Salopek’s beheaded body, found on ISIL-linked social media accounts.
The Croatian Foreign Ministry said it was unable to immediately verify the death.
Last Wednesday, the Sinai Province released a video threatening to kill Salopek, an employee of the French CGG company’s Cairo branch, if their demands are not met.
Croatia is not part of the US-led coalition of 60 nations carrying out or providing ground support to airstrikes on ISIL targets in Iraq and Syria since last August.
Late last year, the US State Department said Croatia had contributed military supplies to the anti-ISIL efforts.
ISIL, a Sunni militant group that controls vast parts of Syria and Iraq, has beheaded hundreds of its hostages, including foreign journalists and contractors, over the past year.
The Sinai Province, initially named Ansar Bayt Maqdis, swore allegiance to ISIL and changed its name in November 2014. It made headlines in June 2015 following a failed suicide bombing of the historic Luxor Temple.
