The withdrawing convoy crossed the Tigris River at the Fishkhabur border post near the Turkish frontier, en route to Erbil, the capital of the semi-autonomous region, which is home to a US base.
US forces also moved into Iraq from Syria through the Sahela border crossing in the northern province of Dohuk, witnesses said. A Reuters news agency cameraman saw more than 100 vehicles crossing.
Residents of a Kurdish-dominated Syrian city pelted US troops with potatoes and shouted expletives at them as they drove through.
A video posted on Monday by the Kurdish news agency showed a convoy of armoured vehicles driving through Qamishli.
Angry residents hurled potatoes and shouted “no America” and “America liar” in English. One vehicle backed up over the pavement, in an attempt to get away from the people.
“Just like rats, America, run away. Long live the resistance and long live the YPG,” a man holding a photo shouted, referring to the Kurdish People’s Protection Units militia.
US defence chief Mark Esper said on Monday some American troops will remain in parts of northeast Syria near oilfields alongside the YPG-led Syrian Democratic Forces (SDF) to ensure the oil does not fall into the hands of the Islamic State of Iraq and the Levant (ISIL or ISIS) group.
“We presently have troops in a couple of cities located right near that area. The purpose is to deny access – specifically revenue to ISIS and any other groups that may want to seek that revenue to enable their own malign activities,” Esper told reporters in Afghanistan.
“There has been a discussion about possibly doing it [keeping some troops]. There has been no decision with regard to numbers or anything like that,” he added.
The US announced the withdrawal of 1,000 American soldiers deployed in northeast Syria on October 13, the fifth day of Turkey’s offensive against the YPG, which Ankara considers a “terrorist” group.
On October 7, US soldiers withdrew from the Turkish border in northern Syria on orders of President Donald Trump, opening the way for Turkey’s offensive against Kurdish forces.
The offensive was paused on Thursday by a fragile ceasefire negotiated by Washington.
On Sunday, an AFP correspondent saw a convoy of more than 70 armoured vehicles flying the American flag and escorted by helicopters drive past the Syrian town of Tal Tamr carrying military equipment.
In the past week, US forces have withdrawn from three other bases in Syria, including the key town of Manbij and another near Kobane close to the Turkish border.
The US military presence at several bases across Iraq is controversial with numerous political groups and pro-Iran Shia militia demanding their expulsion.
The fight in Syria against ISIL – once spearheaded by the American allied Syrian Kurds cast aside by Trump – will be undertaken by US special forces, possibly from neighbouring Iraq, Esper said. His comments to reporters travelling with him were the first to specifically lay out where American troops will go as they shift from Syria.
Trump nonetheless tweeted: “USA soldiers are not in combat or ceasefire zones. We have secured the Oil. Bringing soldiers home!”
Trump’s top aide, asked about the fact the soldiers were not coming home as the president claimed they would, said: “Well, they will eventually.”
Acting Chief of Staff Mick Mulvaney told Fox News Sunday that “the quickest way to get them out of danger was to get them into Iraq”.