A nurse in Germany has been arrested on suspicion of poisoning five premature babies with morphine, said the police on Thursday. They added that the children had all survived. 

The woman was taken into custody on Wednesday after investigators found a syringe containing breast milk and traces of morphine in her locker at University Hospital in Ulm, southern Germany, police chief Bernhard Weber told reporters. 

The five babies were between one day and one month old. They were injected with morphine on 20 December in a shared room at the hospital. They all suddenly developed breathing problems "at almost the same time" in the early hours of the morning. 

"Only because of the immediate action taken by the staff could the five lives be saved," said Weber at a press conference. 

Doctors do not think the babies have suffered any lasting harm. 

The young nurse taken into custody has not yet been formally charged, but is set to face five counts of attempted manslaughter, according to Ulm prosecutor Christof Lehr. 

The nurse denies poisoning the babies. 

Initially, hospital staff suspected the infants had caught an infection, "but this could be ruled out after urine tests," said Weber. 

The tests showed traces of morphine, despite two of the babies not being prescribed the heavy painkiller as part of their care. The hospital notified the police following this discovery. 

A syringe "filled with breast milk" was found after a search of the lockers of the employees on duty around the time of the incident. 

Tests then "confirmed the terrible suspicion that the syringe contained morphine," said Lehr. 

An arrest warrant was then issued for the woman who owns the locker on suspicion of "attempted manslaughter."