Syria will respond to a Turkish aggression on any part of its territory with "all legitimate means" available, President Bashar al-Assad said on Thursday according to state media.
The comments come after an agreement between Damascus and the Kurdish-led Syrian Democratic Forces to fend off an assault launched by Turkey on northeastern Syria last week.
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US Vice President Mike Pence met President Recep Tayyip Erdoğan in Turkey on Thursday on a mission to persuade him to halt an offensive against Kurdish fighters in northeast Syria, but Turkish officials said the action would continue regardless.
The assault has created a new humanitarian crisis in Syria with 200,000 civilians taking flight, a security alert over thousands of Islamic State fighters abandoned in Kurdish jails, and a political maelstrom at home for President Donald Trump.
Trump has been accused of abandoning Kurdish-led fighters, Washington's main partners in the battle to dismantle Islamic State's self-declared caliphate in Syria, by withdrawing troops from the border as Ankara launched its offensive on Oct. 9.
Trump defended his move on Wednesday as "strategically brilliant." He said he thought Pence and Turkish President Tayyip Erdoğan would have a successful meeting, but warned of sanctions and tariffs that "will be devastating to Turkey's economy" otherwise.
The White House released a letter from Trump to Erdoğan from Oct. 9 that said: "Don't be a tough guy" and "Don't be a fool!" Turkish broadcaster CNN Turk said Turkey had rejected Trump's appeal to reach a deal to avoid conflict and the letter was "thrown in the trash".
A Turkish official told Reuters: "The letter Trump sent did not have the impact he expected in Turkey because it had nothing to take seriously.
"What is clear is that Turkey does not want a terrorist organization on its border and the operation will not stop because of the reaction that has been coming."
