WASHINGTON - The United States on Wednesday (June 5) barrelled closer to imposing tariffs on all Mexican imports as high-stakes negotiations at the White House failed to immediately resolve President Donald Trump's demand that Mexico prevent a surge of Central American migrants from flowing across the southwestern border.

Mr Trump declared on Wednesday evening on Twitter that "not nearly enough" progress had been made and warned that "if no agreement is reached, Tariffs at the 5% level will begin on Monday, with monthly increases as per schedule".

New figures released on Wednesday showed that illegal border crossings have risen to a seven-year high, underscoring the roots of the president's rage. But political resistance to Mr Trump's tariff threat has also intensified, with sceptical Republican senators asking to hear directly from the president before he takes an action that could shake the economies of both countries.

Mr Trump, travelling in Europe, had insisted earlier that he was not bluffing, but he also predicted that Mexico would make a deal to avert a series of escalating surcharges on its products.

Mr Marcelo Ebrard, Mexico's Foreign Minister, met on Wednesday afternoon for two hours with Vice-President Mike Pence and Secretary of State Mike Pompeo, hoping to convince Mr Trump's top advisers that Mexico is working aggressively to protect the border.

In his tweet just after 6.30pm local time, Mr Trump said that talks with Mexico had ended for the day and would resume on Thursday.

There was no indication that Mr Trump would be satisfied by anything short of direct evidence that Mexico had completely ended the flow of migration through its country.

The president has repeatedly railed in private and public about what he considers to be a failure of Mexican authorities. He has set a deadline of June 10, saying he will use broad emergency powers to begin taxing all Mexican goods at 5 per cent and to increase the tax to 25 per cent by October if illegal crossings do not completely end - a feat that diplomats, politicians and immigration experts said is wildly unrealistic.

During the White House meeting, which lasted about 90 minutes, Mexican officials described for their counterparts the steps they have already taken to reduce the flow of migration, including deploying additional troops to the border with Guatemala and beefing up the fight against organised crime, according to a senior Trump administration official.

The official said the Mexicans appeared sincere, but Mr Pence concluded that the efforts were insufficient because they would most likely reduce migration only at the margins, instead of the wholesale changes that Mr Trump was looking for.

Mr Pence and Mr Pompeo countered by urging the Mexicans to enter into a "third safe country" treaty in which Mexico would assume responsibility for granting asylum to the migrants, something the Mexicans have steadfastly opposed.

Top legal officials from Mexico are scheduled to meet on Thursday with White House counsel Pat Cipollone to discuss the treaty and other issues, the official said. Diplomats for both countries will meet at the State Department to continue the talks.

Wednesday's announcement by Customs and Border Protection of a significant surge in border crossings was meant to put pressure on the Mexican government to meet Mr Trump's demands. More than 144,200 migrants were arrested and taken into custody along the southwestern border in May, a 32 per cent increase from April and the highest monthly total in seven years. Most crossed the border illegally, while about 10 per cent arrived without the proper documentation at ports of entry along the border.

"Look, the drugs that are coming in, the people that are coming in unchecked, they're swamping our border," Mr Trump said during a meeting with the Irish Prime Minister in Shannon, Ireland. "Mexico can stop it. They have to stop it. Otherwise, we just won't be able to do business. It's a very simple thing."

In his tweet, the president added that border arrests were so high "because of Mexico & the Democrats in Congress refusing to budge on immigration reform".

In a news conference on Wednesday at the Mexican Embassy, Mr Ebrard said he was optimistic about a reaching a resolution before the tariffs go into effect on Monday.

"We have the opportunity to share our point of view, explain why the Mexican position, that we are following regarding this issue, and tomorrow we are going to follow the talks", he said.

He added that the dialogue focused on Mexico's proposals on migration, rather than the tariffs.

While Mr Trump insists he is committed to imposing tariffs, the president faces intense opposition not only from Democrats, but also from business executives, economists and members of his own party, who say tariffs are the wrong approach to dealing with immigration issues. Republican senators have been mobilising to prevent the White House from moving ahead with tariffs, warning Mr Trump that they are almost uniformly opposed to his plans to tax Mexican imports.

At a lunchtime briefing on Tuesday, Senator Mitch McConnell, the majority leader, told administration lawyers that Republican senators needed to hear directly from Mr Trump before he slapped tariffs on Mexico, according to two Republican officials familiar with the discussion.

In the week since Mr Trump announced his tariff threat on Twitter, administration officials have said the Mexican government must secure its own border with Guatemala, through which many migrants travel on their way to the United States. They have demanded that Mexican officials crack down on transnational gangs that facilitate migrant travel. And they have insisted that Mexico agree to take in all of the asylum-seekers who would otherwise claim refuge in the United States.

But officials have provided few other specifics about how Mexico could meet those goals quickly enough to stave off Mr Trump's anger. Mexico has already tried to secure its own southern border and has long fought transnational gangs.

In the past six months, the Mexican government has deported more than 80,500 migrants back to their homes in Central America and elsewhere, according to government data. During the same period, Mexican authorities detained about 400 people accused of trafficking migrants. And nearly 25,000 migrants applied for refuge in Mexico in the first five months of 2019.

"The Mexican government could interdict more migrants for sure, but they can't just flip a switch and turn off the flow," said Mr Kevin Appleby, a veteran expert on migration.

In Mexico, there is popular support for treating Central American migrants humanely, and many Mexicans do not want to see President Andrés Manuel López Obrador bow to the wishes of his counterpart in the United States.

"He's between a rock and a hard place. He doesn't want to be seen as being a toady to the United States," said Dr Doug Massey, a professor of sociology at Princeton University and a director of the Mexican Migration Project. "He doesn't want to violate the human rights of Central Americans, who after all are simply escaping terrible conditions."