BERLIN, June 20 (Xinhua) -- German Chancellor Angela Merkel on Wednesday said migration is the greatest European challenge at the moment, calling for a common instead of unilateral solution in the bloc.

Attending an event in Berlin commemorating the victims of flight and expulsion on the World Refugee Day, Merkel said Germany stood by both its obligation to protect people fleeing war and terrorism and its commitment to keeping Europe together.

"Migration is a European challenge; perhaps our biggest challenge right now. It is about the cohesion of the European Union (EU)," said Merkel.

The German Chancellor said the EU has to manage and organize migration, which must follow clear rules, and common standards in the EU -- who will come and who can stay; and who is not -- should be defined if possible.

"It is in the deepest interest of our countries to hold Europe together. It is of great interest to answer the big questions of foreign, refugee and migration policy together. It would not be good if everyone did it at the expense of the other," said Merkel.

Merkel's remarks comes while her party has been in a dispute with the Bavarian ally on whether to introduce tougher refugee policies and border controls, a situation threatening the country's ruling coalition.

Merkel opposed unilateral measures and she expected to find a common European solution during the EU summit on June 28-29 to maintain the cohesion of the bloc.

An informal summit on migration will be convened on Sunday, on which Merkel will discuss with leaders of migration-affected countries including France, Italy, Greece, Bulgaria and Spain, in a bid to find European solutions to the migration and asylum issues ahead of the EU summit.