German Chancellor Angela Merkel announced her intention to seek an end to EU membership talks with Turkey during her sole face-to-face TV showdown with challenger Martin Schulz on Sunday, putting ties with Ankara centre-stage in Germany's election campaign.

In a sharp escalation of an already intensifying diplomatic crisis between two NATO allies with longstanding historical links, Merkel said: "It is clear that Turkey should not become a member of the European Union."

She said she will discuss with EU counterparts to see if "we can end these membership talks", adding that "I don't see (Turkey) ever joining and I had never believed that it would happen."

Germany will also look at imposing "real restrictions on economic contact" with Turkey, as well as possibly issuing a tougher travel warning against travel to the country, she said.

Merkel's tough line comes after Ankara arrested two more German nationals for "political reasons", according to Berlin.

The plunge in bilateral relations began after Germany sharply criticised Ankara over the crackdown that followed last year's failed coup attempt there.

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.

Merkel's strong stance against Turkey appeared to steal the show from Schulz, who had minutes earlier called for an end to the EU membership talks.

Polls published after the televised debate put the chancellor ahead in the crucial clash.

- Merkel ahead in polls -

Merkel's CDU party and their Bavarian CSU allies are commanding a strong 17-percentage point lead over the SPD ahead of the September 24 polls.

But with almost half the voters still undecided three weeks before the elections, the straight-talking Schulz had been pinning his hopes on the prime-time TV showdown, hoping to sway millions to his cause and halt a devastating popularity slide.

A former European Parliament chief, Schulz enjoyed a surge in support shortly after taking the helm of the SPD in January, only to see that initial excitement fade away.

But two separate surveys conducted after the debate put Merkel ahead, with 55 percent saying in the ARD poll that she was more convincing, compared to just 35 percent for Schulz.

An ZDF survey was closer but still had Merkel ahead with 32 percent, against 29 percent who thought Schulz performed better.

During their 90-minute debate, four seasoned TV presenters tossed wide-ranging questions to the two top politicians, ranging from Germany's migration crisis to relations with US President Donald Trump to pensions.

Schulz, who had previously complained that Merkel was lulling voters to sleep with her refusal to engage in combative debate, went on the offensive quickly.

On the hot-button topic of migration, he accused Merkel of failing to coordinate plans with EU neighbours when she decided to open Germany's borders in 2015 to allow in refugees, many from war-torn Syria and Iraq.

But Merkel countered that: "In the life of a chancellor, there comes a time when you have to make a quick decision."

Recounting the situation when hundreds of thousands of refugees were blocked at the Hungarian border, she said she "had no hope that (Hungarian Prime Minister) Viktor Orban would change his mind" over his tough stance against refugees.

Given a minute for a closing speech at the end of the debate, Merkel turned back to her now-infamous slogan "we'll manage this" that had marked the refugee crisis.

"I believe that we will manage this together," she told viewers, as she asked them for their support at the ballot boxes on September 24.

Source: AFP