U.S. troops leaving Syria started crossing into Iraq on Monday, Reuters reports.

The news service reports that one of its staffers saw more than 100 vehicles crossing into Iraq from the northeast tip of Syria. Reuters video also showed armored vehicles carrying U.S. troops crossing into Iraq's northern province of Dohuk. ADVERTISEMENT

Defense Secretary Mark Esper Mark EsperOvernight Defense: Stopgap spending measure awaits Senate vote | Trump nominates former Nunes aide for intelligence community watchdog | Trump extends ban on racial discrimination training to contractors, military Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Official: Pentagon has started 'prudent planning' for full Afghanistan withdrawal by May MORE said Sunday that troops will be relocated to western Iraq, where they will continue to conduct operations to prevent a resurgence of ISIS.

President Trump had said, however, that he was bringing the U.S. troops home after withdrawing them from northern Syria. After Trump's announcement, Turkey quickly launched a military operation in the area targeting Syrian Kurds, who aided the U.S. in the fight against ISIS.

Esper said on Monday that he is discussing an option to keep a small residual of U.S. military force in Syria and that some of the U.S. forces in the area have yet to withdraw, according to The Associated Press.

Esper, who is in Kabul, Afghanistan, said he had not reached a final decision on that option or presented it to Trump, according to AP.

Trump’s decision to withdraw troops from Syria faced pushback from lawmakers on both sides of the aisle. Even Sen. Lindsey Graham Lindsey Olin GrahamLincoln Project mocks Lindsey Graham's fundraising lag with Sarah McLachlan-themed video The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-S.C.), who is typically a supporter of the president, harshly criticized the decision.

Vice President Pence announced last week that Turkey had agreed to a five-day cease-fire, which would expire late Tuesday. But reports have indicated that fighting has continued in some parts of the border amid the announced truce.