BRUSSELS (Reuters) - Austria and Slovenia cannot send asylum seekers back to the country where they first entered the European Union during the 2015 migrant crisis, an adviser to the EU's Court of Justice said on Thursday, in a case likely to have wider repercussions for the bloc.

A Syrian man who sought asylum in Slovenia and an Afghan family who did the same in Austria have asked the European Court of Justice to rule on the decision of authorities that they should be returned to Croatia, their point of entry into the EU.

EU law, known as the Dublin Regulation, typically says the country an asylum seeker first enters the bloc is responsible for handling their application.

However, the court was asked whether that rule still applied under the exceptional circumstances of 2015, when more than a million people fleeing war and poverty in the Middle East and Asia arrived, prompting a number of EU countries to close their borders and to argue over how the refugees should be handled.

The Advocate General, whose advice is usually followed by Europe's top court, said the unprecedented inflow of migrants and the lack of clear guidance in the Dublin Regulation on how to handle such a situation could leave EU border states - such as Croatia or Greece - unable to cope.

"In the Advocate General's view, the regulation was simply not designed to cover such exceptional circumstances," the court said in a statement.

The recommendation is likely to have implications for other asylum seekers, many of whom only lodged their requests when they reached their final destination, often Germany or Sweden rather than southern nations such as Greece where they first entered the EU.

(Reporting by Robert-Jan Bartunek; editing by Philip Blenkinsop and Gareth Jones)