Turkey's Erdogan says Germany 'aiding' terror'

President Recep Tayyip Erdogan on Friday accused Berlin of "aiding and harbouring" terror after German authorities banned rallies courting support from Turkish expatriates for a constitutional change to expand Erdogan's powers.

Ankara and Berlin were locked in acrimony and Erdogan also claimed a detained correspondent working for Germany's Die Welt newspaper was a  spy, an accusation Germany said "makes no sense."

"They need to be put on trial for aiding and harbouring terror," Erdogan said, criticising German authorities who had allowed outlawed Kurdish leaders to speak but had blocked rallies where Turkish ministers sought to address the Turkish community. 

The president also claimed that Die Welt correspondent Deniz Yucel, who was detained on Monday on terror-related charges, was a "German agent."

Turks vote on April 16 on whether to create a presidential system -- a change that the government says will ensure political stability, but which critics say will drag Turkey into one-man rule. 

In the runup to the referendum, controversy has flared over politicians' trips to Germany, where they have been seeking "Yes" votes from the millions of people of Turkish descent.

One such event was a rally by Prime Minister Binali Yildirim in the western city of Oberhausen. 

On Thursday, local authorities blocked rallies by two more Turkish ministers, prompting a furious response from Ankara which promptly summoned the German envoy to protest. 

An incensed Erdogan, who was prevented from adressing a rally in Cologne by video link after a failed coup in July last year, lashed out at Germany. 

"They allow Cemil Bayik to speak from (the) Kandil" mountains in Iraq, he said, referring to the outlawed Kurdistan Workers' Party (PKK) leader. 

Earlier, Foreign Minister Mevlut Cavusoglu also accused German officials of failing to "honour democracy" and of allowing "terrorists" to speak but denying the same right to Erdogan.

"They don't want Turkey to campaign here, they are working for a 'No'," he said. "They want to get in the way of a strong Turkey."

German Chancellor Angela Merkel rejected Turkey's accusations, saying the decisions were "taken by municipalities, and as a matter of principle, we apply freedom of expression in Germany".

Cavusoglu and German Foreign Minister Sigmar Gabriel, speaking by phone, agreed to meet next Wednesday, a senior Turkish official said.