Washington (CNN) White House economic adviser Larry Kudlow said Sunday he doesn't believe President Donald Trump will make good on his threat to impose a 25% tariff on cars made south of the border if Mexico doesn't help stem the flow of undocumented immigrants into the United States, saying it's just a "warning" that should be heeded.

"Regarding the threat of tariffs, (the) President believes that the border issue, the border security, the wall, the breakdown in immigration are very important issues -- including the drug trafficking. So he is saying, if need be, he may have to take the emergency steps to deal with that. He is not about to do it. He has pulled back a wee bit. He's just issuing a warning, and if you know this President as I do, you better listen to him," Kudlow told CNN's Jake Tapper on "State of the Union."

The threat from Trump came last week after he walked back his threat to close the US-Mexico border in an effort to quell immigration into the US. After saying Thursday he would give Mexico a year to step up, he backpedaled a day later, tweeting Friday that Mexico is "for the first time in decades" apprehending illegal immigrants before crossing the border.

"However, if for any reason Mexico stops apprehending and bringing the illegals back to where they came from, the U.S. will be forced to Tariff at 25% all cars made in Mexico and shipped over the Border to us. If that doesn't work, which it will, I will close the Border," he wrote on Twitter.

"This will supersede USMCA," he added, referring to his pending US-Mexico-Canada Agreement. The ratification of the deal could be undermined by the tariff threat, which Kudlow said was Trump "just making a very strong statement."

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