United States military personnel left north-east Syria earlier this month (Picture: US Army).
The Turkish president has announced that the country's offensive in north-east Syria has started.
Tayyip Erdogan said on his official Twitter account that the operation, named 'Peace Spring', has begun.
He said the operation aims to eradicate "the threat of terror" against Turkey.
Earlier, Turkish television reports said jets had bombed Syrian Kurdish positions across the border from Turkey.
The move follows the removal of some US troops, who fought alongside Kurdish forces in north-east Syria against so-called Islamic State (IS).
The Syrian Democratic Forces (SDF) claims IS group has targetted military positions in Raqqa.
A spokesman for the Turkish president said earlier that, if Kurdish militants in the region do not "defect", Ankara would have to "stop them from disrupting" counter-IS efforts.
"YPG militants have two options: They can defect or we will have stop them from disrupting our counter-ISIS efforts," they added.
Turkey are NATO allies to the US, but consider the Kurdish forces to be terrorists.
Tensions have risen in the north-eastern region of Syria following the removal of US military personnel.
In response to concerns, Mr Trump threatened to "obliterate" the Turkish economy if the NATO nation acted in an inhumane way.
The US president has said the US "does not endorse" Turkey's assault on Syria, and called the operation "a bad idea".
The SDF have called upon the international community "for the implementation of a No Fly Zone as was done in the past for the people of Iraq."
Fears that US President Donald Trump's decision to remove military personnel from north-eat Syria could leave the SDF vulnerable to attack from Turkey or a resurging IS have led to criticism.
A former British SDF fighter labelled the move a "betrayal", while retired US military officer General David Petraeus raised concerns about America's "level of commitment" to partners and allies.
The United Nations Security Council will hold a meeting behind closed doors on Thursday to discuss the situation in Syria.
UN Secretary General Antonio Guterres "has called on all parties to exercise maximum restraint" concerning the situation in northeastern Syria, a statement released by his spokesperson said.