Relations have been strained between Ankara and Berlin since the crackdown that followed last year's failed coup in Turkey that has seen more than 50,000 people arrested.
Turkish European Union Affairs Minister Omer Celik said discussion over "ending negotiations with Turkey is an attack on the EU's founding values".
In a TV showdown with challenger Martin Schulz on Sunday ahead of a September election, Merkel said it was "clear that Turkey should not become a member of the European Union".
Her strong stance against Turkey appeared to steal the show from Schulz, who had minutes earlier called for an end to the EU membership talks.
Celik hit back at German politicians using "careless language" and "trying to give orders to EU institutions... they think the EU is the 'United States of Germany'".
He added the attitude of some German politicians was to "build a Berlin Wall with bricks of populism", heavily criticising Schulz but only directly accusing Merkel of taking a position against Turkey to "avoid falling behind".
Earlier, Turkish President Recep Tayyip Erdogan's spokesman said attacks on Turkey were a "surrender to populism and marginalisation (and) hostility (which) only fuels discrimination and racism".
"Attacking Turkey-Erdogan and ignoring Germany's and Europe's fundamental and urgent problems are a reflection of a lack of vision," Ibrahim Kalin said on Twitter.
The escalating tensions have split the Turkish community in Europe's top economy, the largest Turkish diaspora abroad, which is a legacy of Germany's "guest worker" programme of the 1960s and 70s.
Yucel has now spent over 200 days in custody ahead of a trial on terror charges.
Last week two more German citizens were arrested, but the German foreign ministry Monday said one of them had been freed without restrictions, according to his lawyer.
The ministry said it was still seeking independent confirmation of the release, but it brings the total number of Germans in Turkish custody now to 11.
EU foreign policy chief Federica Mogherini at the Bled Strategic Forum in Slovenia said Turkey remained a partner in the region and was still a candidate country.
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