Former German chancellor Angela Merkel has argued that international cooperation, not the closure of internal European borders, is the key to addressing irregular migration, reported dpa.

Speaking at a special event during the Lit.Cologne literature festival in Cologne on Monday evening, Merkel said that combatting illegal migration requires collaboration with countries of origin and transit, rather than focusing on internal European borders.

"I don't think the issue can be resolved at our internal borders," Merkel said. "And if it can, it will only be at the very high price of endangering freedom of movement in Europe."

Merkel's comments came as the conservative Christian Democrats (CDU), along with its Bavarian sister party the Christian Social Democrats (CSU) have announced plans for a de facto admissions freeze for illegal migrants, should the conservatives win the next election, expected on February 23.

The draft manifesto section on migration, seen by dpa, states that those entering from another EU member state or the Schengen Area and wishing to apply for asylum in Germany will be turned back at the borders.

The executive boards of the CDU and CSU are set to approve the election programme on Tuesday in Berlin. CDU leader Friedrich Merz, the favourite to be Germany's next chancellor, and CSU chief Markus Söder - the premier of Bavaria - are then expected to present the manifesto.

Former CDU chancellor Merkel presented her book "Freedom: Memories 1954-2021" in Cologne and spoke about her decision to accept what became some 1 million Syrian refugees into Germany. When she read a passage about this, the audience applauded.