"We officially announced that we will start a military operation to the east of the Euphrates", Mr Erdogan said in a speech in the central province of Konya.
All US State Department personnel are being evacuated from Syria within 24 hours, a US official told Reuters, after the White House said it had started withdrawing US forces.
"AGAIN, TO OUR VERY FOOLISH LEADER, DO NOT ATTACK SYRIA - IF YOU DO MANY VERY BAD THINGS WILL HAPPEN & FROM THAT FIGHT THE U.S. GETS NOTHING!" he declared in an all upper-case salvo on Twitter. The developments come after the US State Department late on Tuesday signaled it would support the sale of a American missile defense system to Turkey - an effort that had been held up by Ankara's decision to buy a Russian system.
Republican Senator Lindsey Graham, a Trump ally, said the president's decision was shortsighted.
"We have defeated [the Islamic State] in Syria, my only reason for being there during the Trump Presidency", Trump wrote in a morning tweet just hours after the Wall Street Journal first reported that 2,200 United States troops would soon leave Syria.
Graham warned that a US withdrawal would leave Kurds, who have been a valuable ally in the fight against ISIS, at risk.
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Earlier in the day, officials in the Trump administration told United States media that they were considering a total withdrawal of USA forces from the Middle Eastern country.
Before his election, the real estate tycoon was firmly in the anti-war camp and opposed any United States military action against President Bashar al-Assad.
The partnership with Syrian Kurdish and Arab fighters (the Syrian Democratic Forces) is credited with virtually eliminating IS after it overran large swathes of Syria four years ago, imposing a brutal rule on nearly eight million people across Iraq and Syria.
Trump has pledged to wrap up the US military campaign against the Islamic State (ISIS) in Syria, often writing on Twitter that the USA was moving rapidly to defeat the group.
His tweet followed a series of news reports that the US was preparing to withdraw its troops from Syria.
"We're not going to leave as long as Iranian troops are outside Iranian borders and that includes Iranian proxies and militias", he said.
Much of the US campaign in Syria has been waged by warplanes flying out of Qatar and other locations in the Middle East.
It had been thought USA defence officials wanted to stay to ensure the militant group could not rebuild.
Trump has long promised to conclude the campaign against the Islamic State, also known as Isis, and has questioned the value of costly and risky military missions overseas.
"Even if this does get overturned or significantly revised, it does undermine the already limited leverage the USA has in Syria", Frances Brown, a former National Security Council director under both President Barack Obama and Trump, told Al-Monitor.
Trump has previously lambasted his predecessor, Barack Obama, for the withdrawal of U.S. forces from Iraq that preceded an unravelling of the Iraqi armed forces. Iraqi forces collapsed in the face of ISIS's advance into the country in 2014.
"If we withdraw then who fills the vacuum, who is able to stabilize and that is the million dollar question", said Andrew Tabler, a Syria specialist at the Washington Institute for Near East Policy think-tank.
"I don't understand where this is coming from", he said.
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It would also be nice to see a more competitive side to the sport, which usually sees one side dominate the entire season. The team is looking at Charles Leclerc from Ferrari, Marcus Ericsson or Antonio Giovinazzi who is a reserve for Ferrari.