Twelve Polish pilgrims died on Saturday and 30 others were injured when a bus  veered off a highway and crashed into a ditch in northern Croatia, authorities said.

"They were pilgrims heading for Medjugorje," a Catholic sanctuary in neighbouring Bosnia that draws hundreds of thousands of visitors every year, Polish Prime Minister Mateusz Morawiecki tweeted.

The death toll rose from 11 after one of the injured died in hospital, Interior Minister Davor Bozinovic told a press conference in the northern city of Varadzin.

Eighteen of the 30 hurt, who were all taken to hospitals in the area, were in a serious condition, said Maja Grba Bujevic, head of Croatia's emergency services.

Sixteen emergency medical teams had been dispatched to the crash site.

"All the victims (are) Polish citizens," said Lukasz Jasina, a spokesperson for Poland's foreign ministry.

The bus was registered in Poland, th spokesperson told the TVN24 channel in Warsaw.

Polish Health Minister Adam Niedzielski and Deputy Foreign Minister Marcin Przydacz were expected to arrive in Croatia later in the day.

"A serious accident took place around 5:40 am (0340 GMT). The bus deviated and fell into a ditch off the highway," police spokesman Marko Muric had told AFP.

The crash occurred about 60 kilometres (40 miles) from Zagreb.

Croatia is one of Europe's top tourist destinations, with millions flocking this summer to it stunning Adriatic coast.

About 580,000 Polish tourists visited Croatia between January and July this year.

In July, 2021, ten tourists from Kosovo died in a bus crash on a highway in eastern Croatia.