White House Press Secretary Karoline Leavitt said Tuesday the first flights of undocumented immigrants from the U.S. to Guantánamo Bay in Cuba were underway—following an escalation of arrests across the country as part of what President Donald Trump has promised would be the “largest deportation operation” in U.S. history.

The Trump administration’s first flights carrying detained migrants to Guantanamo Bay are set to depart on Tuesday after the president ordered a large-scale effort to prepare for thousands of prisoners to be sent to the US naval base in Cuba.