Turkey on Wednesday said it would consider a "legitimate target" any group that comes to the aid of Kurdish militia in Afrin in northern Syria.
The threat from presidential spokesman Ibrahim Kalin came a day after Syrian pro-regime forces entered Afrin, only to come under fire from Turkish forces.
Last month Ankara launched a cross-border air and ground offensive backing Syrian rebels 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-regime forces in Afrin.
"Every step taken in support for the YPG terror organisation would mean (any forces intervening on the Kurdish militants' side) are on the same level as terror organisations. 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 Islamic State extremist group in Syria, sparking anger in Ankara.
Turkey has called for Syrian President Bashar al-Assad's ouster and supported opposition fighters 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.