Talking to reporters in western Mugla province, Yildirim said: "Unfortunately, one of our two ATAK helicopters on a mission in Afrin region has crashed. Two heroic members of Turkish Armed Forces were martyred."
Yildirim said it remains “unclear" why the helicopter went down and added that an investigation had been launched.
"We do not have any exact evidence and document regarding whether it was downed due to external intervention," he said.
In a statement, Turkish General Staff said one of the ATAK helicopters crashed at around 1 p.m. (1000GMT) during ongoing operation in Afrin.
Turkey on Jan. 20 launched Operation Olive Branch to remove PYD/PKK and Daesh terrorists from Afrin.
According to Turkish General Staff, the operation aims to establish security and stability along Turkish borders and the region as well as to protect the Syrian people from the oppression and cruelty of terrorists.
The operation is being carried out under the framework of Turkey’s rights based on international law, UN Security Council’s decisions, self-defense rights under the UN charter and respect for Syria's territorial integrity, it said.
The military has also said that only terrorist targets are being destroyed and "utmost importance" is being given to not harm any civilians.
Afrin has been a major hideout for the PYD/PKK since July 2012 when the Assad regime in Syria left the city to the terror group without putting up a fight.