The Christian Social Union (CSU) - the Bavarian sister party to Merkel's Christian Democrats (CDU) - has long criticised Merkel for allowing more than a million refugees into Germany over the last two years.

The CDU and CSU are meeting in Munich to discuss how to defeat a resurgent Social Democratic Party (SPD) in September's federal election.

Merkel and CSU leader Horst Seehofer are due to present their joint election strategy.