Authorities in southwest Germany arrested an intensive care nurse on Wednesday, suspecting she had attempted to kill five newborn babies in her charge with morphine, police in the city of Ulm said.

The babies, at least some of them prematurely born, fell simultaneously ill in the early hours of December 20, suffering from shortness of breath.

Thanks to prompt medical intervention, all recovered and are not expected to face long-term harm.

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"In the early hours, five infants, aged between one day and one month, suffered shortness of breath almost simultaneously," Ulm police chief Bernhard Weber told a news conference on Thursday.

Hospital authorities first suspected an infection, but urine tests disproved that theory, while at the same time revealing traces of morphine in all five infants - even though two of them had not been prescribed the drug.

Morphine is routinely stocked at neonatal departments to treat the withdrawal symptoms of babies born to mothers who are addicted.

Investigators were able to conclude from the time of the symptoms that the poisonings had happened during a night shift. They later found a syringe containing morphine-infused baby milk in the locker of one of the nurses on that shift.

The nurse, who was not identified by authorities but described as "young", was arrested after the syringe was found. She denies charges of serious bodily harm and attempted killing.