German Chancellor Angela Merkel admitted on Monday that mistakes had been made in the refugee crisis but ruled out any major change in course despite her conservative Christian Democrats (CDU) suffering heavy losses in a string of state elections.

Merkel told a press conference following a fresh debacle for the CDU in Sunday's Berlin city state that "not everything had been done properly" following her controversial decision a year ago to open her nation's borders to allow refugees stranded in Hungary to travel to Germany.

"If I could, I would take the time to rewind many years so as to better prepare myself and the government and all those in responsible positions for the situation in late summer 2015 that took us rather by surprise," said Merkel.

About 1 million refugees - many fleeing wars in Africa and the Middle East - entered Germany last year after borders were opened, unleashing political tensions in the country, including in her conservative political bloc.

"In the balance it was absolutely right," to provide refuge for the waves of migrants, Merkel said in a lengthy statement on the refugee crisis.

"But," she said, "it ultimately meant that for a time we did not have sufficient controls," the chancellor and CDU chief said.

"No-one wants a repetition of this situation, including me," she said, accepting responsibility for Sunday's vote, which was the CDU's worst ever result in the city.

She went on to say that if the direction of her refugee policy had been a cause of the CDU's recent poor performance then she planned to strive to explain her government's stance on the issue much better.