Germany's Social Democrats raised pressure on conservative Chancellor Angela Merkel on Sunday, saying if she could not resolve a row with Turkey over access to the Incirlik air base, German troops should move.

Merkel's defense minister, tacitly admitting the possibility said she had been looking for other locations and hinted that Jordan could be one.

Turkey, which has refused permission for German lawmakers to visit their troops at Incirlik, has said Berlin is free to move its soldiers from the base. That would, however, be a significant snub to a NATO ally.

Already strained bilateral ties have deteriorated further over Incirlik where roughly 250 German soldiers are stationed as part of the coalition against Islamic State militants.

"If Mrs Merkel doesn't succeed at the NATO summit on Thursday to get Turkey to change course, we need alternative bases," Thomas Oppermann, head of the SPD parliamentary group told Bild am Sonntag.

The SPD, or Social Democrats, trail Merkel's conservatives in polls four months before the national election. It is desperate to score points with voters on issues other than social justice, its main focus in the last couple of months which has so far failed to resonate.

Defense Minister Ursula von der Leyen is looking at alternatives, including Jordan and Cyprus, and said on Saturday she had been impressed with a possible base in Jordan but stressed the government had not yet made a decision.

Merkel is vulnerable on relations with Turkey as critics accuse her of cosying up to President Tayyip Erdogan, who last month won sweeping new powers in a referendum, as she needs his help to control the flow of migrants to western Europe.

The SPD, junior partner in Merkel's right-left coalition, also tried at the weekend to raise its profile on European issues.

Leader Martin Schulz, a former president of the European Parliament, has tried to ally himself with new pro-EU French President Emmanuel Macron and on Saturday said he would model his campaign on the Frenchman's.

On Sunday he told a rally in Bavaria it was time for a "new German-French initiative for a socially fair Europe of growth."

A week after a disastrous election defeat in Germany's most populous state, an Emnid poll showed the gap widening between Merkel's conservatives, up 1 percentage point at 38 percent, and the SPD, down 1 point at 26 percent.