U.S. officials are set to meet with Mexican negotiators Thursday for a second day of immigration talks, with President Donald Trump's threat to slap 5% tariffs on all Mexican imports looming over the discussions. The duties are set to kick in in just four days. Trump threatened to implement rising tariffs on all Mexican imports if the country did not stop the increasing tide of undocumented migrants crossing the southern U.S. border. The first day of high-level talks at the White House, which included Vice President Mike Pence and Mexican Foreign Minister Marcelo Ebrard, ended without a deal Wednesday afternoon, with both sides failing to bridge their differences on the immigration issue. Staff-level meetings between U.S. and Mexican officials were expected to continue at the White House, an administration official told CNBC on Thursday morning. There were also meetings between the two countries at the State Department. Trump, who was in Europe meeting with world leaders and commemorating the 75th anniversary of the D-Day invasion, told reporters earlier Thursday that "I think a lot of progress was made yesterday, but we have to make a lot of progress."

But when Trump announced the tariffs in a surprise tweet last Thursday, it was not clear what his administration specifically wanted Mexico to do to curb illegal immigration. Trump initially said the U.S. would slap the tariffs on "until such time as illegal migrants coming through Mexico, and into our Country, STOP." But a White House statement released just after that tweet said that "if the illegal migration crisis is alleviated through effective actions taken by Mexico, to be determined in our sole discretion and judgment, the Tariffs will be removed." On a call with reporters that night, Trump's acting chief of staff, Mick Mulvaney, said the White House didn't have a "specific percentage" or level of reduction in mind. On Wednesday, White House trade advisor Peter Navarro offered his own proposals in a television interview, saying the tariffs "may not have to go into effect at all" if Mexico agreed to make certain concessions during the talks. Navarro said Mexico would need to crack down on asylum seekers, strengthen enforcement of its own southern border with Guatemala and address government corruption at Mexican immigration checkpoints. "That's it. That's what we're looking for," Navarro claimed. But it was unclear what level of influence, if any, Navarro had on the negotiations. He did not appear to be present for the talks Wednesday.