The US has begun its operation to pull troops out of Syria, sending naval vessels and back-up forces to the region to assist with the withdrawal.
"Out of concern for operational security, we will not discuss specific timelines, locations or troops movements", Ryan said in a statement to the Associated Press.
"On Thursday, some American forces withdrew from the Rmeilan military base in Hasakeh province", Rami Abdel Rahman, the head of the Britain-based monitoring organisation, said. A convoy of about 10 armored vehicles and some trucks pulled out of Rmeilan, Syria, heading for and into Iraq.
Trump's decision to remove troops from Syria sent shockwaves through the embattled region and led to the resignation of Defense Secretary James Mattis.
Recent reports have indicated the U.S. is considering leaving some troops in place at the al-Tanf base in southern Syria, which is seen as a key bulwark against Iran.
The removal of the equipment in recent days was first reported by CNN, which quoted an administration official with direct knowledge of the operation as saying it signaled the beginning of United States withdrawal from the Middle Eastern country.
Syrian Kurds demonstrate against threats from Turkey to carry out a fresh offensive following the USA decision to withdraw their troops in the Syrian Kurdish-majority city Qamishli, on December 28, 2018.
Turkish Defence Minister Hulusi Akar has reiterated during his meeting with Joseph Dunford, the chairman of the US Joint Chiefs of Staff, Turkey's call on the US to promptly withdraw the US-backed Kurdish People's Protection Units (YPG) from the Syrian city of Manbij, the Turkish Defence Ministry said in a statement on Tuesday.
Late Sunday, Trump tweeted that Erdogan had assured him that any remaining IS fighters in Syria will be eliminated.
The US president concluded: "Our troops are coming home!"
Turkey considers America's Kurdish allies of the YPG militia - who were vital in the ground war against ISIS - a terrorist group. Other military officials suggested that the withdrawal of troops would be completed more slowly when the battle against ISIS was over. Analysts argued it would be wrong for Trump to abandon USA allies the Kurds, who have been instrumental in fighting ISIS, because Turkey would likely launch an attack against them.
After initially tweeting about the decision to bring back USA troops "now", Trump this week said "we will be leaving at a proper pace while at the same time continuing to fight ISIS and doing all else that is prudent and necessary!"
Foreign Minister Mevlut Cavusoglu went so far on Wednesday as to say that if the USA withdrawal from Syria was delayed, "then we will put this decision into practice", suggesting Turkey could go after the Kurds in short order. Once Trump announced his intention, Erdogan said Turkey would delay any operation until all USA forces left.