WASHINGTON (AP) -- A top White House official said Sunday that President Donald Trump is "deadly serious" about slapping tariffs on imports from Mexico but acknowledged there are no concrete benchmarks being set to assess whether the U.S. ally was stemming the flow of migrants enough to satisfy the administration.
"We intentionally left the declaration sort of ad hoc," Mick Mulvaney, the acting White House chief of staff, said on "Fox News Sunday."
"So, there's no specific target, there's no specific percent, but things have to get better," Mulvaney said. "They have to get dramatically better and they have to get better quickly."
He said the idea is to work with the Mexican government "to make sure that things did get better."
Trump claims Mexico has taken advantage of the United States for decades but that the abuse will end when he slaps tariffs on Mexican imports next week in a dispute over illegal immigration.
Trump tweeted Sunday: "America has had enough."
The president said last week that he will impose a 5% tariff on Mexican goods on June 10 to pressure the government of Andres Manuel Lopez Obrador to block Central American migrants from crossing the border into the U.S.
Trump said the import tax will increase by 5% every month through October, topping out at 25%.
"He's absolutely, deadly serious," Mulvaney said.
Mexican officials are due to meet later this week with Secretary of State Mike Pompeo in a bid to come to a resolution.
Mulvaney, who also spoke Sunday on NBC's "Meet the Press," said Mexico could take various steps to decrease the record numbers of migrants at the border.
He suggested the Mexican government could seal its southern border with Guatemala, crack down on domestic terrorist organizations and make Mexico a safe place for migrants seeking to apply for asylum.
"There are specific things that the Mexicans can do," he said on Fox.
Economists and business groups are sounding alarms over the tariffs, warning they will hike the costs of many Mexican goods Americans have come to rely on and impair trade.
But Mulvaney downplayed those fears, saying he doubts business will pass on the costs to shoppers. "American consumers will not pay the burden of these tariffs," he said.
He also suggested the tariffs were an immigration issue, separate from the trade deal the United States is trying to negotiate with Mexico and Canada.