At least 20 people have reportedly died and 24 others injured in a bus accident in Iran, according to an Iranian newspaper.

The bus was reportedly heading to Gonbad from the capital Tehran when the accident took place, according to the Entekhab newspaper.

Gonbad is about 500km (310 miles) northeast of Tehran, near the border with Turkmenistan.

An image posted on social media showed the bus lying on its side, with shattered windows, by the side of the road.

The Tehran-based Shargh Daily newspaper quoted authorities as saying that the accident was caused by a defect in the vehicle's brake system.

ILNA news agency also quoted authorities from Mazandaran Province as saying that mechanical failure was the cause of the deadly incident.

A video of the aftermath of the crash showed emergency personnel struggling to extract survivors from the mangled bus, while medics tried to load an injured victim into an ambulance.

UNICEF said Iran's rate of traffic accidents is 20 times the world's average, as the country suffers from poor roads and lax law enforcement.

At least two fatalities were also seen placed in a bodybag.

According to a May 2017 report by the Tehran-based newspaper Financial Tribune, more than 20,000 people are killed and 800,000 injured annually in road accidents in Iran.

More details to follow.