ISTANBUL (Reuters) - Turkey s European Union Affairs Minister Omer Celik said on Monday that any talk of ending his country s negotiations for EU accession amounted to an  attack on Europe s founding principles . His comments came a day after German Chancellor Angela Merkel said it was clear that Turkey should not join the EU and that she would talk to other EU leaders about ending its stalled accession process. Merkel s main challenger in Germany s Sept. 24 national election, Martin Schulz, has also promised to push for an end to Turkey s EU negotiations if elected chancellor.  They are building a Berlin wall with bricks of populism,  Celik tweeted. Turkey will  keep going with its head held high as a European country and a European democracy,  he said. 