About a dozen children were on the bus as seven passengers were saved and taken to a hospital, the interior ministry says.
At least 45 people have died in an overnight bus accident on a highway south of the Bulgarian capital.
The cause of Tuesday’s accident on a highway 40km (26 miles) from Sofia was yet to be determined.
“Forty-five or 46 people were killed,” Nikolay Nikolov, head of the Fire Safety and Civil Protection Department under the Ministry of Interior, told public broadcaster BNT.
“Seven passengers were saved” and taken to a hospital in the capital, he added.
According to the bTV channel, 12 children were on board the bus, which was going from Turkey’s main city of Istanbul to Skopje in North Macedonia.
Prime Minister Zoran Zaev, who reportedly wanted to visit the scene of the crash, said the victims were probably fellow Macedonians.
“We don’t know if all the victims are from North Macedonia, but we assume so because the bus is registered in the country,” he told Nova TV.
Television footage showed the bus charred and gutted by fire in the middle of the highway.
“We have an enormous tragedy here,” Bulgarian interim Prime Minister Stefan Yanev told reporters.
Interior Minister Boyko Rashkov said: “People are clustered inside and are burnt to ash.”
“The picture is terrifying, terrifying. I have never seen anything like that before,” he told reporters at the site.
Bulgarian investigative service chief Borislav Sarafov said four buses from a North Macedonian travel agency had entered Bulgaria late on Monday from Turkey.
“Human mistake by the driver or a technical malfunction are the two initial versions for the accident,” he said.
The accident happened on Struma highway about 45 km (28 miles) west of Sofia around 2:00 a.m. (0000 GMT).
North Macedonian Foreign Minister Bujar Osmani said the coach party was returning to Skopje from a weekend holiday trip to Istanbul.
“I am terrified. This is such a huge tragedy,” Zaev told private television channel BTV.
Bulgarian investigative service chief Borislav Sarafov said four buses from a North Macedonian travel agency had entered Bulgaria late on Monday from Turkey [File: Stoyan Nenov/Reuters]