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%.

But the president has been here before, issuing high-stakes threats over his frustration with the flow of migrants only to later back off. They include his threat earlier this year to seal the border with Mexico.

Republicans on Capitol Hill and allies in the business community have signaled serious unease with the tariffs that they warn will raise prices for consumers and hurt the economy. Some see this latest threat as a play for leverage and doubt Trump will follow through.

GOP Sen. John Kennedy of Louisiana, on Sunday called the tariffs a "mistake." He said it's unlikely Trump will actually impose them.

The president "has been known to play with fire, but not live hand grenades," Kennedy said on CBS's "Face the Nation."

"It's going to tank the American economy," he said. "I don't think the president's going to impose these tariffs."

Mexican officials are due to meet later this week with Secretary of State Mike Pompeo in a bid to come to a resolution.

"I think what the president said, what the White House has made clear, is we need a vast reduction in the numbers crossing," Kevin McAleenan, acting secretary of the Department of Homeland Security, said on CNN's "State of the Union."

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.

Mulvaney insisted that Trump's threat is real. "He's absolutely, deadly serious," Mulvaney said.

Economists and business groups are sounding alarms over the tariffs, warning they will hike the costs of many Mexican goods that 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.

The tariff threat comes just as the administration has been pushing for passage of the United States-Mexico-Canada Agreement, which would update the North American Free Trade Agreement.

Several top GOP lawmakers have expressed concerns that Trump's tariff threat could upend that deal. The chairman of the Finance Committee, Sen. Chuck Grassley of Iowa, said last week the tariffs would "seriously jeopardize" passage of the USMCA, which needs approval in Congress.