The official quoted by CNN would not describe exactly what the cargo was or how it was being transported.
When did the withdrawal start?
The U.S. military command in Baghdad, which is managing the counter-IS campaign in Iraq and Syria, said Friday that it "has begun the process of our deliberate withdrawal from Syria", adding that, for security reasons, it would not reveal timetables, locations or troop movements.
"For purposes of operational security, we will not discuss specific troop movements or timelines", he added.
The Britain-based Syrian Observatory for Human Rights, which monitors the conflict in Syria through a network of activists on the ground, said the withdrawal began on Thursday night.
Meanwhile, Israeli Prime Minister Benjamin Netanyahu confirmed Sunday that Israel had struck hundreds of Iranian and Hezbollah targets in Syria over the weekend, including a weapons facility, as the Israeli Defense Forces announced the discovery of a sixth and final tunnel dug by the Lebanese militant group for cross-border attacks. It says about 10 armored vehicles and other equipment arrived at a US base in al-Rmelan, in al-Hasaka province.
A second United States defence official told AFP the military had conducted a number of preparations for a deliberate withdrawal.
President Trump shocked allies and faced strong criticism at home last month when he ordered USA forces to immediately begin withdrawing from the approximately 30% of Syria controlled by the YPG-led Syrian Democratic Forces (SDF) alliance.
More than 2,000 US troops are serving in Syria. But that time frame has since been extended several times. He said many details around the troop withdrawal still need to be ironed out, but that he was optimistic a "good outcome" could be reached between Turkey and Kurdish groups in Syria.More news: Andy Murray’s career recognised with Wimbledon statue
More news: Resident Evil 2: 9 Things To Look Forward To
More news: #SaveRahaf: Activists' lightning campaign made Saudi teen's flight a global cause
"CJTF-OIR has begun the process of our deliberate withdrawal from Syria", said Col. Sean Ryan, a spokesman for the anti-ISIS coalition, citing the Department of Defense.
A line of USA military vehicles drive through a checkpoint of the Internal Security Forces in Manbij as they head to their base on the outskirts of the northern Syrian city on December 30, 2018.
But even as the SDF advance, there are growing concerns about their safety.
A Kurdish-led group, the Syrian Democratic Forces, is now flushing the jihadists from the very last pockets of land they control in the Euphrates River Valley.
She also confessed that the terrorist organization forcefully recruited underage children and sent them to fight against Turkish soldiers during the Operation Olive Branch. "Our preparations are continuing intensively".
At the same time, Trump urged the Kurd-dominated Syrian Democratic Forces (SDF), which the U.S. trained and armed for years, not to "provoke" Turkey.
"What we're still pursuing in these military-to-military conversations are assurances and protocols and procedures so that everybody feels comfortable with how this is going to happen", Bolton said.
"And we're hoping those discussions, which will continue next week, will produce results that are acceptable on both sides".
Trump's national security advisor John Bolton on Sunday announced conditions for a withdrawal that appeared to delay it indefinitely.