German police said on Wednesday they had arrested a Serbian man accused of two murders and a series of other violent attacks against the elderly.

The suspect is accused of killing a 77-year-old woman in December and an 89-year-old woman in January in or near the southwestern city of Schwaebisch Hall.

Investigators also believe the same assailant is behind the armed mugging of an 83-year-old man last month and possibly a further killing in 2020.

"We were able to arrest a 31-year-old Serbian citizen yesterday morning," regional police chief Reiner Moeller said, adding that any motive was still unclear.

"He has declined to answer questions about the crimes."

The suspect told investigators that he only arrived in Germany in December, with his family, prosecutor Harald Lustig told a news conference.