WASHINGTON (AP) — Top officials from the U.S. and Mexico will begin talks Monday in a scramble to fend off President Donald Trump’s threat of devastating tariffs on imports from the southern ally and meet his demand for fewer migrants at the border.

Trump is in London for a long-planned overseas trip, leaving others to stem a potential trade crisis. It’s unclear what more Mexico can do — and what will be enough — to satisfy the president. Trump’s Republican allies warn that tariffs on Mexican imports will hit U.S. consumers and harm the economy.

The president all but taunted negotiators for a quick resolution. “Mexico is sending a big delegation to talk about the Border,” the president tweeted Sunday. “Problem is, they’ve been ‘talking’ for 25 years. We want action, not talk.”

On Monday, Mexican Economy Minister Graciela Marquez plans talks with Commerce Secretary Wilbur Ross. Two days later, delegations led by Secretary of State Mike Pompeo and Foreign Relations Secretary Marcelo Ebrard will also meet in Washington.

Trump has been here before, issuing high-stakes threats, only to back off come crunch time. But a top White House official warned that the president was “deadly serious.” Trump is threatening 5% tariffs on Mexican imports starting June 10.

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. His frustration with the flow of migrants is nothing new, but it’s a subject he often returns to, as he did last week after special counsel Robert Mueller’s rare public statement on the Trump-Russia report.

The president said last week that he will impose the tariffs 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%. It swiftly refocused attention on the border issues.

Mick Mulvaney, the acting White House chief of staff, said on “Fox News Sunday” that the president is “deadly serious.”

Still, Mulvaney acknowledged there are no concrete benchmarks being set to assess whether the U.S. ally is stemming the migrant flow enough to satisfy the administration. “We intentionally left the declaration sort of ad hoc,” he said.

“So, there’s no specific target, there’s no specific percentage, but things have to get better,” Mulvaney said. “They have to get dramatically better and they have to get better quickly.”

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 and top Republicans warned it could derail that effort.

GOP Sen. John Kennedy of Louisiana, called the tariffs a “mistake” and said it was unlikely Trump would impose them.

Republicans on Capitol Hill and GOP allies in the business community have expressed serious unease with the tariffs. Some see this latest threat as a play for leverage and doubt Trump will follow through. Earlier this year Trump threated to seal the border with Mexico only to change course.

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.”

Republicans have repeatedly tried to nudge Trump away from trade wars and have specifically questioned the White House’s ability to rely on executive authorities to impose some of them as national security issues.

At the same time, Trump’s efforts to revamp immigration laws have drawn little support in the Congress.

“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 appeared 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.

Economists and business groups are sounding alarms over the tariffs, warning that they will impair trade and increase the costs of many Mexican goods that Americans have come to rely on.

But Mulvaney played down those fears, saying he doubts business will pass on the costs to shoppers. “American consumers will not pay for 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.

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

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