"A serious accident took place around 5:40 am (0340 GMT). The bus deviated and fell into a ditch off the highway," Croatia's police spokesman Marko Muric said.
Police, firefighters and medical teams were deployed to the site of the accident which occurred near Breznicki Hum in the direction of the capital Zagreb, police also said.
Poland’s health minister, foreign ministry top officials, medics and psychologists are now on their way to the site, Prime Minister Mateusz Morawiecki wrote in a Twitter post on Saturday.
He added that the travelling Poles were pilgrims heading to Medjugorje.
"I want to extend my deepest sympathies to the families of the victims," he also wrote.
Croatia has attracted millions of tourists this summer, drawn to its stunning Adriatic coast. About 580,000 Polish tourists visited Croatia between January and July this year.
In July, 10 tourists from Kosovo died in a bus crash on a highway in eastern Croatia.
(mo)
Source: IAR, AFP