Mr Trump's withdrawal of the few dozen USA troops in the border area was seen as a "stab in the back" by the Kurdish-led Syrian Democratic Forces (SDF) and drew condemnation even from the president's Republican allies.
The move comes just days after U.S. President Donald Trump's announcement that American troops would withdraw from the region and hand control to the Kurds' sworn enemy, the Turkish government. "All preparations for the operation have been completed", the Turkish Defense Ministry said.
U.S. officials meanwhile reportedly said they had no plans to take control of prisons holding Islamic State (Isil) suspects if their Kurdish guards withdrew, further raising fears that jihadists could escape amid the offensive.
Turkey has reportedly begun shelling US -backed Kurdish positions in northern Syria, while President Donald Trump insisted the United States had not abandoned the Kurdish militias who would be targeted in the assault after he recently said he pulled back 50 to 100 "special operators" from the area.
McGurk, who resigned shortly after Trump announced US troops would pull out of Syria late December, said that Turkey does not have the intent, desire, nor the capacity to manage detainees the SDF holds at al Hol camp, which Pentagon officials warn is the "nucleus for a resurgent ISIS".
Iran, a key backer of the Syrian government, also opposed a Turkish incursion, with Foreign Minister Mohammad Javad Zarif urging "respect for Syria's territorial integrity" in a call to Ankara.
Turkish troops and the newly-regrouped Syrian National Army (SNA) began Operation Peace Spring in northeastern Syria, Turkey's President Recep Tayyip Erdogan said on Wednesday.
"Last night, we passed dozens of military vehicles heading in this direction - APCs [armoured personnel carriers], what we believe to be trucks carrying ammunition, and flatbed trucks transporting tanks", he said.
USA troops were pictured with armored vehicles near the city, known in Kurdish as Sere Kaniye and located in the northeastern province of Hasakah, just days before.
Russian Foreign Minister Sergey Lavrov, meanwhile, accused the US of playing "very unsafe games" with the Syrian Kurds.
Given that those same Kurdish groups have fought alongside USA forces against the Islamic State movement in Syria, Trump's decision was seen by prominent figures of his own Republican party as a betrayal. Concerning Syria, he abruptly announced a full withdrawal of US troops last December - only to be convinced to leave a rump force of about 1,000 troops to work with the Kurdish-led forces in the battle to defeat ISIS. Experts who spoke to Al-Monitor said the incursion would likely be limited and face stiff resistance from the Kurdish-dominated Syrian Democratic Forces (SDF), which have received backing from the Pentagon.
The top Russian diplomat stressed that both Damascus and the Syrian Kurds have said they are "open to dialogue, and we will do our best to help launch talks on this subject".
ISIS launched three suicide attacks against its positions in Raqqa, said the SDF. There was no immediate word on casualties.
This article has been updated with additional reports by the Syrian Observatory for Human Rights.
Turkey has rid an area of 4,000 square km (1,544 square miles) in Syria of terrorist groups in two separate cross-border operations.
Raqqa is being Silently Slaughtered, an activist collective, reported an exchange of fire and a blast.