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

The jet was on a flight from Barcelona to Boston with 296 passengers when it was diverted, according to media reports.

The civil protection service said 14 passengers needed help after the plane landed safely at the airport in northern Portugal. Eight passengers who were on the Airbus A330 were treated on the tarmac at the airport, a spokesperson for the civil protection authority told Reuters.

Four passengers were taken to the hospital suffering from smoke inhalation but they were only "lightly" affected, a spokesperson said.

The cause of the incident was not immediately known, according to the civil protection service.

Delta recently had four emergency landings in a one-week span — two due to reports of cabin fumes and two due to apparent pressurization issues.

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