An Afghan man died in Bulgaria after being indirectly hit by a bullet of border police on Thursday, authorities say.

The incident took place around 22:00 local time (EEST), the Interior Ministry's Chief Secretary, Georgi Kostov, has announced.

Pretrial proceedings have been launched.

It happened as border police detected the movement of 54 people trying to enter Bulgaria near the town of Sredets, in the country's south-east, not far from the border with Turkey. After shooting in the air as a warning sign, a bullet "rebounded" hitting one of them in the back of the neck by mistake, Kostov has explained, without elaborating.

According to the version he put forward, a border police team was deployed to the site of the incident after movement in the area, presumably of the migrants, had triggered the integrated border protection system.

A correspondent for the Bulgarian National Television near the border with Turkey reported that, according to latest reports, border police fired the warning shots as the group was moving in an area known to be the "connection point" where smugglers would load migrants on buses.

"These people were illegally crossing the border... These offenders has been detained," Kostov later said at a press conference. Border police officers, in his words, said the group of migrants had "resisted" and this has prompted police to fire "warning shots."

Asked whether a people smuggler has been detained - as the group were trying to enter in a rough terrain area - he said police are yet to establish more details.

"Imagine how late in the night a group of 40 [sic] people is moving toward you," Kostov explained.

The migrant died from his wounds while an emergency team was fighting for his life. Other reports suggest he died while being taken to hospital.

Prosecutors said at the press conference "all versions" about the death of the migrant were being looked into.

An official with Bulgaria's prosecuting authority added none of the people detained had proper documents. Alongside Kostov she made clear it was so far early to say whether a Bulgarian man, possibly as part of a smuggling ring, was among the migrants detained.

Kostov added, after being asked by reporters, that migrants from Afghanistan (such as the victim and the others detained) are normally extradited under Frontex rules.

After the development Bulgaria's Prime Minister Boyko Borisov promptly left an EU summit dedicated to the migrant crisis to return to Bulgaria, according to state-run BTA agency.

Sky News reports EU Council President Donald Tusk confirmed that Borisov had informed him of the incident.

Despite the low migratory pressure on Bulgaria compared to neighbors such as Greece, Macedonia and Serbia, the influx at the border with Turkey has increased over the past few weeks.

EU Commission President Jean-Claude Juncker told public broadcaster BNR after news of the incident emerged that the Commission was "with Bulgaria" and that the development was "yet another argument about how important our discussion is this evening." Leaders had gathered to discuss more adequate measures to tackle the migrant crisis.

Turkey, which has borders with EU members Bulgaria and Greece, demands that additional funding be allocated to it to help it manage the crisis more effectively, with the number of migrants in the country having surpassed 2 million.