(BSS/AFP) - A Delta Air Lines jet made an emergency landing at Porto airport in Portugal Sunday after smoke was detected in the cabin and four passengers were treated in hospital, the civil protection service said.

The jet was on a flight from Barcelona to Boston in the United States with 296 passengers when it was diverted, media reports said.

The civil protection service said 14 passengers needed help after the plane landed safely at the airport in northern Portugal.

Four were taken to hospital suffering from smoke inhalation but that they were only "lightly" affected, the spokesperson added.

The cause of the incident was not immediately known, the service said.