POTSDAM, Germany (Reuters) - A German man was jailed for life on Tuesday for murdering two young children last year, including a 4-year-old refugee boy whom he had kidnapped from Berlin's main migrant registration center.

The man, identified only as Silvio S., confessed last year to abducting, sexually abusing and killing Mohamed Januzi, a Bosnian boy whose family was seeking asylum in Germany, prosecutors said.

He also admitted kidnapping a six-year-old boy called Elias near the child's home in Potsdam outside Berlin and murdering him in July 2015, the lawyers added. The disappearance of Elias had triggered widespread media attention.

The court in Potsdam said the 33-year-old had acted in a "very cruel" way toward the two children and ruled out the possibility of parole after 15 years due to the severity of the offense.

However, the judge did not order preventative detention - an indeterminate sentence handed to individuals who are considered a danger to public safety - following the recommendation of an expert who said the man did not have an inclination to commit similar crimes in the future.

Silvio S., 33, remained silent for most of the trial but expressed his remorse to the relatives of his victims in a short statement.

"There are no words in the world that could describe how sorry I am. If I could make it undone, I would. I can't even forgive myself," German media reported him as saying.

On Tuesday, he remained silent while the verdict was read, occasionally shaking his head and later crying, said Khubaib-Ali Mohammed, lawyer for the victims.

The Bosnian boy disappeared on Oct. 1 while waiting outside Berlin's central registration office for migrants with his mother and two siblings.

Silvio S. was arrested at the end of October after his own mother told police he had told her he was involved. The child's body was later found packed in cat litter in the boot of his car.

German prosecutors said the suspect had admitted to taking Mohamed to his flat and sexually abusing him. He strangled the child the following morning when he wouldn't stop crying. During questioning, Silvio S. also confessed to the murder of Elias.

(Reporting by Caroline Copley and Reuters TV; Editing by Andrew Heavens/Jeremy Gaunt)