Six pelicans have been found dead at the Srebarna nature reserve in northeastern Bulgaria on Monday, BTA news agency reported.

The cause of the death wasn't immediately known.

The find takes to 27 the dead bodies of pelicans found in the nature reserve, a World Heritage Site located on a freshwater lake of 900 hectares on the border with Romania.

The bodies of 21 dead pelicans were found in Srebarna last week. Bird flu (H5N1) was identified as the cause of their death.

Srebarna, supplied by seasonal floods from the River Danube is the breeding home of 99 species of birds, 24 of which are rare or endangered. Some 80 other bird species migrate and spend winter there.

A team of investigators from the Bulgarian Academy of Sciences used a camera-equipped drone to find whether other birds from endangered species have also died, BTA said

Veterinary and sanitary authorities have imposed restrictions on transportation of poultry meat and eggs in nearby villages and towns to prevent the spreading of the disease. Hunting in a 10-kilometre restricted area around Srebarna has also been prohibited.