Ruthless gunmen have massacred six men and injured another 10 people during a football training session in the Brazilian Amazon.

Gunman armed with rifles stormed a training session held on a football field inside the Urban Social Centre (CSU) in the Compensa district of Manaus.

Brazilian media have reported that the gunmen slaughtered their victims after separating the children from adults.

Amongst those shot dead was 17-year-old Jose Diego Sena Serrao and Michael de Sena Passos, 33, who was a volunteer coach watching the training session.

The killers reportedly fled on two pick-up trucks with two people dying instantly while four passed away in a nearby hospital.

The black-clad gunman wearing masks also injured a 16-year-old boy who is said to be in a critical condition in hospital.

The other dead men have been named locally as David Costa da Silva, 27, Ronaldo Oliveira de Souza, 23, Edilson Xavier Diniz Junior, 24 and Rodrigo de Oliveira Souza, 24.

Homicide and Detention Police (DEHS) head Juan Valério told Globo News said that the shooting on Tuesday (12 December) may have been motivated by a war between drug gangs.

"It was specifically targeted against a football team from the Compensa area, which may have involvement in the crime," he said. "Some of the people who died were involved in drug trafficking."

Of those shot some were said to belong to an amateur football team called T5 Jamaica and local TV stations showed images of the dead covered in blood laying on the football field.

Reports say that one of the escaped vehicles has been recovered and police are investigating a link to two of Brazil's most violent criminal gangs.

The city of Manaus, the capital of the Brazilian state of Amazonas, has suffered from a turf war between the home-grown Family of the North drug gang and the rival Sao Paulo-based First Capital Command (PCC).

The gangs have become famous after prison massacres that have left dozens dead in Brazil's over-populated and brutal prisons.

The murders have led authorities to increase security in the city with Regional Public Security Minister Bosco Saraiva saying the deaths point to "a war between rival drugs gangs".