ZAGREB (AFP) - Prime Minister Zoran Milanovic said Wednesday he was unable to confirm the death of a Croatian hostage that the Islamic State in Iraq and Syria (ISIS) group claimed to have beheaded, but that he feared the worst.

"At this moment we cannot confirm with 100 per cent certainty that this is true and I don't know if we will be able to confirm that in the coming days, but what we see does not look good, looks horrible," Milanovic told reporters.

The Islamic State group claims to have beheaded Tomislav Salopek, who was abducted last month in Egypt, posting a purported picture of the victim's body on ISIS-affiliated Twitter accounts.

The picture's authenticity could not be immediately verified.

If confirmed, this would be the first time a Croatian citizen has been murdered by Islamic militants.

However, Milanovic said his government would not give up as long as there was hope.

"We will not stop searching (for Salopek) as long as the slightest hope exists," he said.

In a separate statement, Croatian President Kolinda Grabar-Kitarovic echoed the premier's insistence that they would not yet give up, adding: "At these moments of deep uncertainty, I express sympathy to the family and friends."

The government, Milanovic said, was keeping in contact with foreign institutions and leaders to seek their advice.

He added that Croatia was "at the full disposal" of the Salopek family.

"This is an attempt by some people to destroy our way of life. If that prevails, it will not be good," he added.

However, he said Croatia "will not take part in fight operations against the so-called Islamic State".