Skopje (AFP) - At least 13 people were killed and more than 30 others injured when a passenger bus overturned on a highway in North Macedonia on Wednesday, health minister said.

Speaking to AFP, Minister Venko Filipce added that "the number of dead may increase because there are people in surgery and in critical condition" following the accident which happened around 20 kilometres (12 miles) from the capital Skopje. Around 50 people were on the bus.

"Thirteen people were killed," Filipce told reporters. "There are more than 30 injured."

Ilir Asani, head of the emergency centre at Skopje's main hospital, told local media that at least "five people were severely injured, including a pregnant woman".

A public prosecutor said it had launched a probe into the accident that occurred "when a bus of the company 'Durmo Tours'... crossed the barrier on the opposite side of the highway, then veered off the road and overturned near the village of Laskarci".

Festim Rusani, 42, was on his way home to Tetovo when he saw the overturned bus.

"I called ambulance and police immediately. With neighbours I helped three people to get out of the bus. But for the rest it was very difficult at first, they were all stuck under the bus," he told AFP.

"There was this girl, young woman, alive while we were trying to help her, she was stuck... She lost her legs but she was still breathing. She died there. It was so traumatic," he added.

Prime Minister Zoran Zaev said "there are no words that can console for the severe traffic accident that was fatal for many of our fellow citizens."

"We compassionate with loved ones for their losses. We mobilise all the strengths and capacities to help everyone whose lives are in life-threatening condition or with injuries to provide them as soon as with the necessary help and return to their loved ones," Zaev wrote on his Facebook page.

Since Tuesday the former Macedonia has been known by its new name the Republic of North Macedonia following a historic agreement with neighbouring Greece