Local media reports say a woman attacked by a US citizen near Neuschwanstein Castle in southern Germany has died while her companion remains in hospital.

One of two female tourists attacked by a man near Neuschwanstein Castle in southern Germany has died, state prosecutors say.

A 21-year-old woman died in a hospital overnight while her 22-year-old companion remained there with serious injuries, the BR public broadcaster reported, citing a representative for state prosecutors in Kempten.

Prosecutors also said a 30-year-old male tourist from the US was in German police custody on suspicion of murder and attempted murder.

The suspect was brought before a magistrate on Thursday, according to police.

BR cited prosecutors as saying the women met a man on a bridge with a view of the world-famous castle and that he led them onto a trail.

BR reported that a scuffle ensued after the man sexually assaulted the 21-year-old.

When the 22-year-old intervened, the man strangled her and threw her 50 metres into a ravine below, the report said.

He then strangled the 21-year-old and pushed her in as well, BR reported.

The suspect was caught after a massive police operation involving 25 emergency vehicles on Wednesday afternoon and taken to a police station in nearby Fuessen, it added.