At least 12 people have been killed and 31 injured after a Polish bus filled with religious pilgrims skidded off a highway in northern Croatia early on Saturday morning.

Some 18 people among the 31 hurt have suffered serious injuries after the bus crashed off the A4 highway in Podvorec, about 30 miles (50km) north of capital Zagreb.

The most likely cause of the crash was the driver falling asleep, state television network HRT reported.

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

“The bus has Warsaw registration plates.”

The passengers were travelling to Medjugorje, a Roman Catholic shrine in southern Bosnia, Croatian Interior Minister Davor Bozinovic said.

Officials from the Polish embassy in Croatia were this morning making their way to the scene, according to a spokesperson for Poland’s Ministry of Foreign Affairs.

Emergency services were deployed after the crash happened around 5.40am near Breznicki Hum, in the direction of Zagreb, police said.

An investigation into the cause of the crash is now underway.

The section of road close to the scene has been closed to traffic to enable passengers to be rescued.

Medjugorje is claimed to be the site of a number of apparitions of the Virgin Mary, who is said to have first been seen by six local children in 1981.

The village, located 12 miles (20km) east of the Croatian border, is now a popular destination for followers of the Catholic faith.

Croatia’s minister of foreign and European affairs, Gordon Grlić Radman, expressed his condolences to the victims of the accident and wished the people hurt a speedy recovery.

“The Croatian people stand in solidarity with Poland and its citizens,” he tweeted.