During the White House briefing later that day, Sanders told reporters the president isn't going "to put an arbitrary timeline" on withdrawal.
"[Trump] is measuring it in actually winning the battle, not just putting some random number out there," Sanders said.
Last week during a speech on infrastructure in Ohio, Trump surprised even the most senior members of his Cabinet by announcing the U.S. planned to get out of Syria "very soon," according to a senior administration official and a U.S. official familiar with the matter.
Then, speaking beside the leaders of Baltic nations at the White House on Tuesday, Trump repeated that call, saying the U.S. will be making a decision “very quickly in coordination with others in the area as to what we'll do” and suggesting that if others, like Saudi Arabia, want the U.S. to maintain a presence in Syria, perhaps they should pay for it.
“I want to get out. I want to bring our troops back home. I want to start rebuilding our nation,” Trump said during the press conference. "It's time. We were successful against ISIS. We'll be successful against anybody militarily, but sometimes it's time to come back home — and we're thinking about that very seriously."
At the same time that Trump was encouraging U.S. troop withdrawal, a top general, diplomat, and development official laid out a strategy for America's involvement in Syria going forward.
"The hard part, I think, is in front of us," Gen. Joseph Votel, the U.S. Central Command chief, told an audience at the U.S. Institute of Peace on Tuesday, "and that is stabilizing these areas, consolidating our gains, getting people back into their homes, addressing the long-term issues of reconstruction and other things that will have to be done."
Votel estimated that more than 90 percent of ISIS's territory in Syria has been reclaimed since 2014.
The general was joined by the U.S. Agency for International Development Administrator Mark Green and the Special Envoy to the Global Coalition to Defeat ISIS Brett McGurk — and all three discussed the importance of their agencies' work to ensure the defeat of ISIS as a key U.S. national security concern.
"We're in Syria to fight ISIS. That is our mission, and that mission isn't over, and we're going to complete that mission," McGurk said.
ABC News' Conor Finnegan and Justin Fishel contributed to this report.