Turkish authorities have arrested at least 666 people since the beginning of a major military operation in northern Syria.

The detained are accused of spreading "terror propaganda".

The Turkish prosecutor has also launched a probe into the conduct of the leader of the main pro-Kurdish party over criticism of the country's offensive in Syria.

Pervin Buldan, Peoples' Democratic Party (HDP) leader, and her fellow legislator Sirri Surreya Onder are also under suspicion of the same, according to Turkish authorities.

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HDP is the only political party in Turkey that opposes Operation Olive Branch, the military action in northern Syria against Kurdish fighters known as the YPG.

"An investigation has been launched into lawmakers Pervin Buldan and Sirri Sureyya Onder by the chief prosecutor for the crimes of carrying out terror propaganda, inciting grudges and enmity in the public," Reuters news agency reported, citing a document from the prosecutor's office.

Buldan, who is also a deputy speaker of the parliament, had been chosen by party delegates to be co-leader of the HDP on Sunday during its congress in Ankara.

The two MPs were being investigated after images of the jailed Kurdistan Workers' Party (PKK) leader Abdullah Ocalan were shown during the congress, according to the prosecutor's office.

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The PKK, designated a "terrorist" group by Turkey, the EU and the US, has fought a bloody, decades-long uprising in the country.



The two parliamentarians are also being probed over allegedly making statements against the Afrin operation during the event.

However, the prosecutor's office did not give specific details about the alleged statements.

The crackdown comes after Turkey opened a new front in the multi-sided Syrian civil war, when it launched an air and ground offensive against YPG fighters on its southern border with Syria last month.

Turkey says the presence of the Syrian Kurdish YPG in the area is a "security threat" to the country.

Turkey sees the YPG - trained, armed and supported by the US to fight against ISIL - as an extension of the outlawed PKK.