MADRID, April 26 (Xinhua) -- Spanish Health Minister Trinidad Jimenez said on Sunday that three Spanish nationals had developed symptoms similar to swine flu a few days after they returned from a Mexico trip.

    Currently, the three were placed in isolation and treated at different hospitals, said Jimenez.

    They were in stable conditions, but it is unclear if they were indeed infected with swine flue and medical personnel are analyzing their cases to come up with definite diagnosis as soon as possible, she added.

    The three came from the northern city of Bilbao, Almansa in the southeast and the eastern city of Valencia, Jimenez said.

    The health minister advised Spanish citizens who had been to Mexico and the United States to have physical check-ups within 10 days upon their return. They should see a doctor immediately if they have any suspicious symptoms.

    The public health authorities are still searching for information about passengers who took the same flight with the three suspicious patients on their trip to Spain.

    The minister said the three were merely individual cases and that the situation has been brought under control. She assured the public not to panic and said the authorities will take all measures to prevent the spreading of swine flu.

    The World Health Organization (WHO) on Saturday urged countries to be alert for unusual flu outbreaks after a deadly swine flu virus has claimed dozens of lives in Mexico and infected at least 11 people in the United States.

    Swine flu is a respiratory disease that infects pigs and does not normally infect humans. But sporadic cases have happened, usually among people who had close contact with pigs.