0415 GMT January 23, 2019
The threat from presidential spokesman Ibrahim Kalin came a day after Syrian pro-government forces entered Afrin, only to come under fire from Turkish forces, AFP wrote.
Last month Ankara launched a cross-border air and ground offensive backing Syrian militants against the Kurdish People's Protection Units (YPG) in Afrin. Turkey says the YPG is linked to the Kurdistan Workers' Party (PKK), which has waged an insurgency against the Turkish state since 1984, and is blacklisted as a terror group by the United States and the European Union.
"Today, tomorrow could they attempt this again? It's possible. But the relevant measures have been taken," Kalin said, after Turkey said it fired "warning shots" at Syrian pro-government forces in Afrin.
"Every step taken in support for the YPG terror organization would mean (any forces taking sides with the Kurdish militants) are on the same level as terror organizations. And for us, that would make them legitimate targets," Kalin told reporters in Ankara.
"It doesn't matter who makes such an attempt, there will be serious consequences."
Syrian state media on Monday said that pro-Damascus forces would head to Afrin to "join the resistance against the Turkish aggression".
The YPG has controlled Afrin since Syrian government forces withdrew from Kurdish-majority areas in the country's north in 2012.
The US has been working closely with the YPG against the Daesh terror group in Syria, sparking anger in Ankara.
Turkey has called for Syrian President Bashar al-Assad's ouster and supported opposition militants throughout the seven-year conflict.
Kalin said Ankara does not have any official contact with Damascus, but that if necessary, Turkish and Syrian intelligence could enter into "direct or indirect" contact.