Journalists accompanied Syrian government forces who entered the center of the city of Manbij, a flashpoint where US troops had previously conducted joint patrols with Turkey. Russian and Syrian flags were flying from a building on the city outskirts, and from a convoy of military vehicles, Reuters reported.
US forces announced they had pulled out of the city.
A week after reversing US policy and pulling troops opening the way for Turkey to attack Washington's allies, Trump announced a package of sanctions on Ankara.
But the measures – mainly a hike in steel tariffs and a pause in trade talks – were less robust than financial markets had expected, and Trump's critics derided them as too feeble to have an impact.
Trump's unexpected decision to withhold support for Syria's Kurds after a phone call with Turkish President Recep Tayyip Erdogan a week ago swiftly upended five years of US policy in the Middle East.
The United States announced on Sunday it was withdrawing its entire force of 1,000 troops from northern Syria. Its former Kurdish allies immediately forged a new alliance with Syrian President Bashar al-Assad's government, inviting the army into towns across the breadth of their territory.
Russian-backed Syrian forces moved swiftly to fill the void left by departing Americans from Manbij, west of the Euphrates River, which Turkey has vowed to capture.
A group of journalists accompanied by Syrian forces journeyed into Manbij city where upon their arrival a group of people gathered, waving the Syrian flag and pictures of Assad.
However the reporters left when gunfire was heard and a group of some 10 young men in Kurdish YPG uniforms began breaking cameras and yelling. Syrian state media said SDF members had opened fire on a march organized by the people of Manbij to welcome the army.
Trump's pullout ends joint US-Turkish patrols of the Manbij area under a deal aimed at persuading Turkey not to invade.
Syrian state television broadcast footage of government troops entering Manbij on Tuesday, under their new deal with the Kurds.
Sanctions ‘falls very short’
Trump has defended his reversal of US policy as part of a plan to withdraw the United States from "endless" wars in the Middle East.
The Democratic Speaker of the House of Representatives Nancy Pelosi said Trump's sanctions were too little, too late.
"His announcement of a package of sanctions against Turkey falls very short of reversing that humanitarian disaster."
Trump's sanctions include reimposing steel tariffs and halting talks on a trade deal. But bilateral trade between Turkey and the United States is small – around a tenth the size of Turkey's trade with Europe. Washington's most effective form of economic leverage would be to hinder Turkey's access to US financial markets, a step Trump has so far avoided.
Turkey says it aims to defeat the Kurdish YPG militia, which it sees as terrorists for their links to separatists in Turkey, and to create a "safe zone" where millions of Syrian refugees can be resettled.
The United Nations says 160,000 people have fled their homes as Turkish forces advance. The Kurdish administration puts the number of displaced at 270,000.
The UN Human Rights office said on Tuesday Turkey could be held responsible for war crimes by forces under its direction.
In a potentially more damaging blow, German carmaker Volkswagen said it was postponing a final decision on whether to build a one-billion-euro (1.1-billion-dollar) plant in Turkey, citing concern over "current developments" after international condemnation of the incursion.
British Foreign Minister Dominic Raab said on Tuesday Britain is to halt new arms export licenses to Turkey as a result of concern over its military operation against Kurdish forces in northeast Syria.
“The UK government takes its arm export control responsibilities very seriously and in this case, of course, we will keep our defense exports to Turkey under very careful and continual review,” Raab told Parliament.
“No further export licenses to Turkey for items that might be used in military operations in Syria will be granted while we conduct that review.”