"We located six corpses which we will remove in the morning. We evacuated 11 wounded firemen on Thursday evening," the commander of the GSS national rescue service, Vinko Prizmic, told AFP.

The victims, most of them aged between 17 and 33, became surrounded by flames when powerful winds changed the direction of the blaze that broke out on Kornat around midday on Thursday, said Croatian fire chief Mladen Jurin.

"They simply had nowhere to escape and died of burns," the head of the rescue team sent to the spot, Stipe Bozic, told national television.

The 11 survivors were rushed to a hospital in the central coastal city of Zadar and five of them were taken to the capital Zagreb where doctors were fighting to save their lives.

The five were in a critical condition.

"They have second and third degree burns which cover 80 to 95 percent of their bodies," said Drasko Boljkovac, a doctor at the Zagreb hospital.

The wildfire, in a national park, was extinguished overnight by an army unit that was later dispatched to the scene.

Croatian Prime Minister Ivo Sanader, who had paid a visit to the injured in Zadar, said a probe would be conducted into the cause of the blaze.

"The government pledges a prompt investigation into the origin of this tragedy, the most serious to have struck Croatian firefighters," he told reporters, adding that several suspected arsonists had been detained.

"We hope that the people responsible will be identified and severely punished," said Sanader.

Eight people suspected of being linked with the fire were detained on Kornat island, police said.

"A probe is ongoing to establish whether they are linked with the fire," police spokesman Krunoslav Borovac said.

The deaths occurred a day after the leader of another crew of firefighters crew died of a heart attack brought on by carbon dioxide poisoning while trying to extinguish a forest blaze on the touristy island of Hvar on Wednesday.

Meanwhile, the government proclaimed for Monday a day of national mourning while Sibenik district where tragedy occurred proclaimed seven days of mourning.

Since June, around 13,000 hectares (32,000 acres) of land have been devastated by some 900 fires mainly along Croatia's Adriatic coastline, according to authorities.

The fires were mainly as the result of a heatwave. So far police have pressed charges against about 30 people suspected of starting the fires, according to the interior ministry.