SOFIA - A man travelling with a large group of refugees from Afghanistan was shot by a border police officer near the southeastern Bulgarian town of Sredets late on Thursday and died on his way to the hospital, a senior interior ministry official said.

"Our border patrol of border guards and police in the area had stumbled on 50 offenders, who illegally entered the country," Georgi Kostov, the chief secretary of the interior ministry, told national radio.

Kostov said the would-be refugees were aged between 20 and 30 were said to be of Afghan origin and were detained in good condition near Sredets, some 30 kms (18 miles) from the Bulgarian-Turkish border.

"They put up resistance during the arrest. One of the officers fired warning shots and, in his words, one of the migrants was wounded by a ricochet and later died," he said.

The prosecutors are investigating the incident - the first since the start of a refugee inflow in the Balkan country two years ago.

Prime Minister Boiko Borisov left an EU summit on refugees in Brussels and returned home after hearing of the incident.

The Black Sea state, a member of the European Union but not of the border-free Schengen Area, has deployed more border police, installed cameras and motion sensors, and is extending a security fence to cover 160 kms (100 miles) of its border with Turkey.

Tens of thousands of migrants, most of them fleeing war and hardship in Syria, are trying to reach Western Europe through the country and its neighbours, Greece, Macedonia and Serbia.

The spokesman for the United Nations' refugee agency UNHCR, Boris Cheshirkov, condemned the use of power against migrants and appealed to Bulgaria to investigate transparently and thoroughly the incident.