LONDON (WASHINGTON POST) - President Donald Trump warned on Tuesday (June 4) he will impose a 5 per cent tariff on Mexican imports starting next week, saying Republican lawmakers would be "foolish" to try to stop him.

Trump's comments suggested imposition of the tariffs June 10 is nearly inevitable - despite pleas from GOP lawmakers to reconsider as they warn of ruinous economic consequences.

Trump said talks with Mexico would continue even as he goes forward with the tariffs aimed at forcing Mexico to take action to stem the tide of Central American immigrants and asylum seekers to the United States.

"It's more likely the tariffs go on and we'll probably be talking during the time the tariffs are on," Trump said during an overseas news conference with British Prime Minister Theresa May.

As for GOP talk of voting on a disapproval resolution to block the tariffs, Trump said: "Oh I don't think they will do that, I think if they did it's foolish."

"There's nothing more important than borders," Trump added.

"I've had tremendous Republican support."

Trump suggested Mexico could take action to stave off the tariffs, and Mexican officials expressed optimism they could reach a deal to head off the penalties.

"Mexico shouldn't allow millions of people to try and enter our country, and they could stop it very quickly and I think they will," Trump said.

"And if they won't, we're going to put tariffs on. And every month those tariffs go from 5 per cent to 10 per cent to 15 per cent to 20 and then to 25 per cent."

High-level delegations from both Mexico and the United States are scheduled to meet on Wednesday in Washington to try to reach an agreement. Trump has threatened to impose the economic penalties unless Mexico significantly reduces the flow of immigrants to the US border.

Trump's tone contrasted with that of Mexico's foreign minister, Marcelo Ebrard, who said during a news conference on Tuesday he thought his country had an 80 per cent chance of a deal.

Mexico has launched a full-court press to try to avert the US tariffs, which would start at 5 per cent and rise each month to a ceiling of 25 per cent unless the country blocks Central American migrants crossing its territory.

Mexico's economy minister, agriculture minister and others are meeting with US counterparts, and delegations of Mexican lawmakers and business leaders are also heading to Washington to warn against the tariffs.

The penalties could severely affect Mexico, which sends 80 per cent of its exports to the United States, its top trading partner.

President Andres Manuel Lopez Obrador told a news conference in Mexico that he was confident that the two sides "will reach an accord before the 10th of June."

Trump told reporters that his top aides would meet with the Mexicans on Wednesday and "we are going to see if we can do something, but I think it's more likely that the tariffs go on."

The tariff threat has so alarmed GOP lawmakers on Capitol Hill that they have begun discussing voting on a second disapproval resolution to overturn Trump's declaration of an emergency at the border.

That could result in an explosive confrontation between Trump and members of his own party, who strongly oppose tariffs as taxes on US consumers.

Senate Republicans will hear from Deputy White House Counsel Patrick Philbin at a party lunch later on Tuesday to discuss the legal rationale for imposing the tariffs.

Mexican officials have indicated they are willing to take new steps to restrain the movement of migrants to the US border, although they have not described them publicly.

The Trump administration has listed three actions it wants from Mexico: strengthening its southern border with Guatemala; stepping up interdiction of busloads of migrants being smuggled through its territory; and agreeing to accept asylum seekers instead of allowing them to proceed to the US border.

Mexican officials have ruled out the third request.

US authorities have detained more than 100,000 migrants along the Mexican border in each of the past two months. Mexico has nearly tripled monthly deportations since the start of the year but is struggling to cope with the rising flow of migrants.