Ankara on Wednesday said it would consider a "legitimate target" any group that comes to the aid of Kurdish YPG militants in Afrin in northern Syria.
The threat from presidential spokesman İbrahim Kalın came a day after Syrian popular forces entered 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," Kalın told reporters in Ankara.
"It doesn't matter who makes such an attempt, there will be serious consequences."
Ankara has called for Syrian President Bashar al-Assad's ouster and supported terrorist groups throughout the seven-year conflict.
During the media briefing in capital Ankara, Kalın dismissed opposition to the operation in Afrin.
He said: "The Republic of Turkey does what is needed for its national interests and we owe no explanation [for Afrin]."
Kalın denied making any official contact with the Syrian government, adding: "But under extraordinary conditions, our intelligence unit may contact [the Syrian authorities] to solve certain issues, directly or indirectly."
