European Union member states should detain migrants who have no case for asylum to prevent them from running away before they are deported, the chief migration official with the bloc's executive arm in Brussels said.

The European Union is pushing to reduce immigration after some 1.6 million refugees and migrants reached its shores via the Mediterranean in 2014-2016. It wants to prevent people from coming and deport more.

"Return rates have to be improved," Migration Commissioner Dimitris Avramopoulos said on Thursday in presenting new proposals by the executive European Commission to increase deportations.

"Member states should also use the possibility to place migrants in detention if there is a risk of absconding and for a sufficient period to be able to complete the return and readmission procedure."

The Commission said the proposals identified areas in which member states could change their current practices to streamline relocations in line with existing laws that are applied differently in various countries.

The Commission said member states should consider longer periods of detention of up to 18 months, although Avramopoulos added that it should only be an option in cases where "migrants are not cooperating" or "there is risk of absconding".

"But it should never be considered something like a concentration camp," he said.