Twelve people have been killed with 31 injured — 18 of them seriously — after a Polish bus filled with religious pilgrims slipped off a road and crashed near Varazdin in northwestern Croatia, authorities said.

Police, firefighters and medical teams were deployed to the site of the accident that occurred at 5:40am on Saturday, near Breznicki Hum in the direction of the capital Zagreb, the police said.

"We have 43 injured people, of them 12 [are] deceased," said Maja Grba-Bujevic, the director of the Croatian Emergency Medicare Institute.

"All the victims are Polish citizens — we can at this point confirm this," a Polish Foreign Ministry spokesman told Polish private broadcaster TVN24.

"The bus has Warsaw registration plates."

Croatia’s state HRT television said the most likely cause of the crash was the driver falling asleep.

The passengers were adult pilgrims who were travelling to Medjugorje, a Roman Catholic shrine in southern Bosnia, Croatian Interior Minister Davor Bozinovic told reporters.

He said that an investigation into the cause of the accident was underway.

The spokesperson for Poland's Ministry of Foreign Affairs, Łukasz Jasina, told Polish news channel TVN24 that survivors would receive support.

“Representatives of our embassy are going to the scene of the accident. The Croatian deputy prime minister and the minister of internal affairs are also going,” Jasina said.

Wires/ABC