The United States and its western allies, not having the stomach for a broader role in the Syrian civil war, have instead restricted themselves to defeating ISIS in Iraq and Syria - a job which is, as President Trump said, now largely complete.
"Timetables or the timing of the withdrawal occurs as a result of the fulfillment of the conditions and the establishment of the circumstances that we want to see", Bolton said.
The United States of America leaves some of the forces in southern Syria for an unknown period of time after withdrawal of the main troops, reports NBC with the reference to its sources in the Pentagon.
"We want to protect the Kurds, but I don't want to be in Syria forever", Trump said last week.
USA defence secretary Jim Mattis and a top United States official in the fight against IS, Brett McGurk, resigned soon after his announcement last month.
"We won", Trump said at the time - a move that both allies and critics warned could upset the balance of power in the Middle East, emboldening Russian Federation and Iran, and threaten what tenuous stability USA -aligned forces had been able to achieve in Syria.
According to the official, the guarantees include giving assurance to avoid military action in Syria unless fully coordinated with Washington.
Bolton said in Israel that the US would pull out only after its troops had rooted out what's left of ISIL in Syria and after the administration had reached an agreement with Turkey to protect Kurdish militias who have fought alongside Americans against the extremists.
National security adviser John Bolton, who is in Israel for talks, has also indicated a more gradual withdrawal.
"But this would mean a YPG pullout from certain areas, which is not clear, especially given both Turkish talks with Russian Federation, but also YPG talks with Assad regime", he said.
The Latest developments from U.S.
The Syrian Democratic Forces in December captured the last town held by IS in Syria, but the militants still hold small pockets of territory in Deir el-Zour province and there are concerns the group would take advantage of an American troop pullout to regroup. Turkey regards the Kurdish fighters we're allied with as separatists and terrorists.
Analysts and military experts, who disagree with Mr Trump, have said the threat posed by Isis still remains.
President Donald Trump made the right call to withdraw us troops from Syria, Erdoğan said.
"The US could use this pull-out process creatively for a long-term strategy as it has leverage on both PKK and Turkey, but at the moment Trump administration is wanting on such long-term thinking", he said.
While the USA announced in December its intentions to pull out of Syria, Russia has been directly involved in the eight-year war with its forces fighting alongside Bashar al-Assad's army since 2015. He said we are leaving. Mr. Trump himself, however, also toured the area in a previous visit.
Turkey has already launched two cross-border operations in Syria - first in 2016 against ISIS in Azaz and al-Bab then in 2018 against the YPG in Afrin.