The comments by John Bolton during a visit to Israel signalled a far more gradual US withdrawal than initially set out by Trump, whose December 19 announcement concerned allies and led to the resignation of defence secretary Jim Mattis.
Trump himself has more recently spoken of "slowly" sending troops home "over a period of time", and Bolton laid out some of the conditions required before it takes place.
"We're going to be discussing the president's decision to withdraw, but to do so from northeast Syria in a way that makes sure that ISIS is defeated and is not able to revive itself and become a threat again," Bolton said when meeting Israeli Prime Minister Benjamin Netanyahu in Jerusalem.
"And to make sure that the defence of Israel and our other friends in the region is absolutely assured, and to take care of those who have fought with us against ISIS and other terrorist groups."
Israel is especially worried over whether the withdrawal will allow its main enemy Iran to expand its presence in the neighbouring country.
Netanyahu has pledged to continue to act against Iran in Syria.
Israel has carried out hundreds of air strikes in Syria against what it says are Iranian military targets and deliveries of advanced weapons to Hezbollah, a Shiite Lebanese militia backed by Tehran.
Earlier in the day, Bolton told journalists travelling with him that conditions such as guarantees on the safety of Kurdish allies must be met before American troops are withdrawn, NBC News reported.
Bolton also said all 2,000 US forces may not be pulled out, according to the report.
He said the withdrawal would take place in northeastern Syria, while some forces could remain to the south at the al-Tanf garrison as part of efforts to counter Iran's presence.
'Condition the withdrawal'
"There are objectives that we want to accomplish that condition the withdrawal," said Bolton, according to NBC.
"Timetables or the timing of the withdrawal occurs as a result of the fulfilment of the conditions and the establishment of the circumstances that we want to see. And once that's done, then you talk about a timetable."
Kurdish-led forces control a large swathe of Syria's north and northeast, some of it seized from the Islamic State group.
Despite backing from the US-led coalition and success in pushing back IS, Kurdish-led forces have at times incurred heavy losses.
A US withdrawal could leave them exposed to an attack by neighbouring Turkey -- where Bolton travels on Monday -- and its Syrian proxies.
The Kurds have invited Damascus to send troops into some of the territory it controls, to act as a bulwark against pro-Turkish forces.
Bolton's visit was part of a US effort to reassure allies after Trump's announcement.
US Secretary of State Mike Pompeo, who also met Netanyahu last week, leaves on Tuesday for an eight-day trip to Amman, Cairo, Manama, Abu Dhabi, Doha, Riyadh, Muscat, and Kuwait City.
Netanyahu used Sunday's meeting to again push for the United States and other countries to recognise Israel's sovereignty over the Golan Heights, the strategic plateau it seized from Syria in the 1967 Six-Day War and later annexed.
He said he planned to take Bolton to visit the Golan before he leaves for Turkey on Monday if weather permits.
"The Golan Heights is tremendously important for our security and I think that when you're there you'll be able to understand perfectly why we'll never leave the Golan Heights," Netanyahu said alongside Bolton.
"And why it's important that all countries recognise Israel's sovereignty over the Golan Heights. I've discussed this with the president."
