Germany saw about 890,000 asylum-seekers arrive last year, many after Ms Merkel decided in September 2015 to let in migrants who were stuck in Hungary.

Numbers have since declined sharply, but Ms Merkel's approach to the migrant crisis has provoked discord within her Christian Democratic Union party, which has seen a string of poor state election results this year.

"A situation like the one in the late summer of 2015 cannot, should not and must not be repeated," Ms Merkel told party delegates at a congress in the western city of Essen.

"That was and is our, and my, declared political aim," she said.

While Ms Merkel has continued to insist that Germany will take in people in genuine need of protection, her government has moved to toughen asylum rules and declare several countries "safe" - meaning people from there cannot expect to get refuge in Germany.

Ms Merkel was a driving force behind an agreement between the European Union and Turkey earlier this year to stem the flow of migrants.

Ms Merkel announced last month that she will seek a fourth four-year term as chancellor in an election expected next September. Her springboard to that run is re-election as the chairwoman of the CDU.

The vote in Essen, where she was first elected chairwoman of the Christian Democratic Union in 2000, offers a test of Ms Merkel's standing with members.

Aside from unhappiness about her migrant policy, some members are grumbling about what is perceived as a wider drift to the left during her 11 years as chancellor.

Polls show a solid lead for the conservatives, though their support is well short of the 41.5% they won in Germany's 2013 election.

They face new competition from the upstart nationalist Alternative for Germany party, which has thrived by attacking Ms Merkel's migrant policies.

She is running unopposed for another term at the CDU's helm. Two years ago, she won the support of 96.7% of delegates, one of her best results.