German Chancellor Angela Merkel has vowed to block Turkey joining the EU, as she pitched for re-election in a TV debate.

In a pivot from her previous position, the centre-right leader declared on Sunday: "The fact is clear that Turkey should not become a member of the EU."

Mrs Merkel pledged to convince fellow European leaders to end the accession talks if returned for a fourth term in the German federal elections on September 24th.

Turkey has been in negotiations to join the EU since 2005, but talks recently grinded to a halt and EU leaders have stepped up their criticism of President Tayyip Erdogan.

Mrs Merkel made the promise after being challenged by her main rival for chancellor, Martin Schulz.

The SDP leader tried to outmanoeuvre Mrs Merkel by saying he would end Turkey's EU talks if he won.

The incumbent cautioned against such a move, saying it would be irresponsible to endanger ties with Turkey at a time when 12 German citizens are imprisoned there on political charges.

"I do not intend to break off diplomatic relations with Turkey just because we're in an election campaign and want to show each other who is tougher," she said.

But Mrs Merkel returned to the issue later in the debate, suddenly throwing her weight behind the idea.

Her Christian Democratic Union (CDU) party has backed the move for years, but Ms Merkel said since Turkey's accession talks began before her premiership, she would respect them.

There was no immediate reaction from Turkey, which is in the midst of a national religious holiday.

Mrs Merkel was some 14 points ahead of Mr Schulz in opinion polls before the debate.

Her overall performance was viewed as more convincing by 55%, compared to 35% for Mr Schulz, in a survey by Infratest Dimap for ARD television.