A German nurse who allegedly killed his victims out of boredom has been charged with 97 murders, potentially making him one of the country's deadliest serial killers.

The nurse, Niels Hoegel, was already serving a life sentence for murdering six patients. He was charged with another 97 murders Monday, prosecutors said in a press release.

He is accused of giving victims various non-prescribed drugs in an attempt to show off his resuscitation skills to colleagues and fight off boredom.

Prosecutors said Hoegel should have been aware that the drugs given to patients in clinics in Delmenhorst and Oldenburg, in northwest Germany, could cause life-threatening cardiac problems.

When patients started suffering the effects of the drugs they were given, Hoegel is accused of failing to help them.

He is accused of killing 62 patients in Delmenhorst and 35 patients in Oldenburg clinics.

Hoegel was previously jailed in 2015 after being convicted of killing six people, leading authorities to investigate hundreds of deaths and exhume bodies of former patients in the clinics where he worked.

In past hearings, NiHoegelels said he felt euphoric when he managed to bring a patient back to life, and devastated when he failed, AFP said.

Police said the final number of murders may never be known as some possible victims were cremated.