The regime of Syrian strongman Bashar al-Assad claimed Monday through state-run media that Turkey began launching aerial attacks across the border, as American troops are obeying President Donald Trump's order to withdraw to the United States.

Trump defended his decision as unconfirmed reports of airstrikes blanketed Twitter, largely traceable back to unverified amateur video. Syrian government-run SANA TV called it 'Turkish aggression' targeting the 'north-eastern countryside' of Syria where some Kurdish forces are based.

WASHINGTON — American forces in northeastern Syria received an urgent, unexpected alert early Monday morning to pull back from their posts.

"They were told, 'We're departing the field,'" said current and former senior U.S. defense officials.

At 3 a.m. local time, the commander of the Syrian Democratic Forces, Gen. Mazloum Kobane, also received a phone call from a senior U.S. official telling him to get on a video teleconference with an American military commander who informed him President Donald Trump had ordered U.S. troops to withdraw.

WASHINGTON—Senate Majority Leader Mitch McConnell urged President Trump to “exercise American leadership” and keep U.S. troops in northern Syria, joining a chorus of Republican and Democratic lawmakers who warned the president against withdrawing from the area.

“American interests are best served by American leadership, not by retreat or withdrawal,” the Kentucky Republican said. He didn’t directly criticize Mr. Trump, but reminded him that the Senate voted in January to keep U.S. troops in Syria, in reaction to a previous withdrawal plan announced by the White House.