Germany's foreign minister warned of a return to nationalism in Europe as several challenges - from uncontrolled migration to eurozone instability - put political unity among EU member states to the test.

Currently posing a threat to the 28-member European Union is the "politics of fear, of isolation and of refusing common European solutions," Frank-Walter Steinmeier said Saturday.

"For a long time it seemed the European project was not in danger. As a result of the financial and migration crises, Europe has gone into a tailspin," he told regional newspaper Neue Osnabruecker Zeitung.

About 1 million people migrated into Europe last year, dividing the bloc as eastern states resist efforts from others such as Germany to forge an EU-wide response to the humanitarian crisis.

Steinmeier warned of rivalry between independent states being whipped up by right-wing populists advocating for an end to the EU, claiming that indicated a "dangerous historical amnesia."

One of Europe's most fervent eurosceptic campaigns is currently taking place in Britain, as the country gears up for an in-out referendum on EU membership in less than seven weeks.

Steinmeier's comments came as strikes paralyzed Greece for a second day Saturday, ahead of a meeting on eurozone finance ministers seeking to stave off bankruptcy in the country.