US-bound migrants detained in Mexico are being transported back to Guatemala as Mexican officials continue efforts to prevent President Donald Trump from imposing tariffs.

Government buses were seen crossing the international bridge between Mexico and Guatemala in Ciudad Hidalgo on Thursday carrying migrants who are being forced to return home to the countries they'd fled from.

The deportation came one day after a caravan with 1,000 people heading north to seek asylum in the US was intercepted by Mexican officials on a highway outside of Tapachula.

The Mexican government is fighting to contain the flow of illegal immigrants amid negotiations with the Trump administration to avoid the implementation of potentially catastrophic tariffs threatened by the president.

A bus transporting migrants detained by the Mexican government is seen passing under a sign that reads: 'Welcome to Mexico' as it crosses a border bridge into Guatemala on Thursday

A man peers out of the window of the bus taking him and fellow asylum seekers back to the Central American countries they were fleeing when detained by the Mexican government

The deportation came one day after a caravan with 1,000 people heading north to seek asylum in the US was intercepted by Mexican officials on a highway outside of Tapachula

Mexico offered to send up to 6,000 members of its national guard to secure its southern border with Guatemala on Thursday - but Vice President Mike Pence says a deal is still not ready.

Pence led an initial round of negotiations in Washington on Wednesday, saying talks were positive but emphasizing the administration still wants Mexico to commit to working harder to combat illegal immigration.

'We welcomed the efforts of the Mexican officials to offer solutions to the crisis at our southern border, but we need Mexico to do more,' Pence said on Thursday.

The Washington Post reported that the United States has proposed deporting Guatemalan illegal immigrants and asylum-seekers to Mexico as part of a deal, but that part of a broader agreement is facing snags.

Trump wants to deport Central American asylum seekers to a third country, and to force them to apply for US asylum at the American embassy of the first 'safe' country they enter after leaving their own.

Current US policy allows anyone who reaches American soil to ask for asylum, virtually guaranteeing they are eventually released into the interior of the country pending hearings that can be years away. A majority don't show up and remain in the US illegally.

President Donald Trump, pictured in Ireland on Thursday, is monitoring immigration negotiations going on between his team and Mexico's, but no one is ready to ink a deal yet

US border authorities last month alone have made 132,887 apprehensions, including a record 84,542 adults and children traveling together and 11,507 unaccompanied minors

Mexico sent a delegation of officials to Washington to try to avert Trump's threats to impose new import tariffs on Mexican goods if the country doesn't do more to keep hundreds of thousands of migrants from pouring through its territory.

Mexico has shown some signs of coming to heel in the wake of Trump's latest tariff threat. The president has warned that 5 per cent border levies on all its exports to the United States will take effect on Monday, and rise to as much as 25 per cent later this year.

The Mexican Finance Ministry said Thursday that it blocked the bank accounts of 26 people for their alleged involvement in human trafficking.

The ministry's Financial Intelligence Unit (FIU) said in a statement that it froze the accounts due to 'probable links with human trafficking and illegal aid to migrant caravans.'

Trump said Wednesday that a 'lot of progress' still had to be made to halt his tariff plan, even as members of Congress in his own party are publicly opposing it.

Underscoring the scope of the border problem, the Department of Homeland Security announced Wednesday that US Border Patrol apprehensions of migrants illegally crossing the border hit the highest level in more than a decade in May.

They made 132,887 apprehensions, including a record 84,542 adults and children traveling together and 11,507 children traveling alone.

Trump, traveling in Europe, tweeted from Ireland that the Washington talks would continue 'with the understanding that, if no agreement is reached, Tariffs at the 5% level will begin on Monday, with monthly increases as per schedule.'

Marcelo Ebrard, Mexico's Secretary of Foreign Affairs, spoke to the media as he left a meeting about tariffs at the State Department on Thursday

Mexican Foreign Secretary Marcelo Ebrard spent several hours at the US State Department Thursday morning, and additional staff-level talks were underway at the White House between Trump's legal counsel and other Mexican aides.

But it remained unclear that any deal could be struck with Trump out of the country, and US officials were preparing for the tariffs to kick in barring major Mexican action.

'We'll see what happens,' Trump told reporters in Ireland before leaving for France to attend a D-Day ceremony. 'But something pretty dramatic could happen. We've told Mexico the tariffs go on. And I mean it, too. And I'm very happy with it.'

Vice President Mike Pence, who led the discussions Wednesday with Secretary of State Mike Pompeo and other US officials, echoed Trump: 'We made clear to them that President Trump is going to continue to stand firm until we bring this crisis of illegal immigration on our southern border to an end.'

Illegal immigration was the signature issue of Trump's presidential campaign and one that he sees as crucial to his 2020 re-election.

Beyond Trump and several White House advisers, though, few in the administration believe imposing tariffs is a good idea, according to officials familiar with internal deliberations.

Those people worry about the negative economic consequences for Americans and believe the tariffs – which would likely spark retaliatory taxes on US exports – would also hurt the administration politically. The officials were not authorized to discuss the matter publicly and spoke on condition of anonymity.

A boy was photographed waiting to be given asylum or a humanitary visa at the immigration office on the Mexico-Guatemala international bridge in Ciudad Hidalgo, Chiapas State, Mexico, on Thursday; Trump would rather see scenes like this play out somewhere other than the US; his team wants the authority to deport asylum-seekers and illegal immigrants to Mexico no matter where their trips originated

The tariffs carry enormous economic implications for both countries, and politically they underscore a major ideological split between Trump and his party. Trump has increasingly relied on tariffs as a bludgeon to try to force other nations to bend to his will, dismissing warnings, including from fellow Republicans, about the likely impacts on American manufacturers and consumers.

Republicans in Congress have warned the White House that they are ready to stand up to the president to try to block his tariffs, which they worry would spike costs to US consumers, harm the economy and imperil a major pending US-Mexico-Canada trade deal.

Democratic House Ways and Means Committee Chairman Richard Neal said he will introduce a resolution of disapproval to stop the tariffs if Trump goes through with his threat, panning it as presidential 'overreach.'

Trump has gone after those who have criticized the tariff threat. A 'lot of people, senators included – they have no idea what they're talking about when it comes to tariffs,' Trump said. 'They have no – absolutely no idea.'