Homeland security secretary Kirstjen Nielsen said American soldiers have “no intention” of shooting migrants who try to cross into the US from Mexico – at least for the time being – but they will all be apprehended, as Donald Trump moves towards a plan to close the border.

The Trump administration on Thursday announced it is planning to dispatch at least 800 active duty troops to the southern border, at the direction of a president who has sought to transform fears about immigration into electoral gains in the November midterms, as a caravan of thousands of migrants makes its way through Mexico.

The US defense secretary, Jim Mattis, is expected to sign an order sending the troops to the border, bolstering national guard forces already there.

And on Thursday evening, in an interview with Fox News while visiting the border in Arizona, Nielsen said: “We do not have any intention right now to shoot at people, but they will be apprehended, however.”

The government is weighing an order to deny even asylum applications at a fortified border, by asserting emergency powers that would almost certainly face challenges in the US courts, the Washington Post reported.

Nielsen added: “But I also take my officer and agent, their own person safety, extraordinarily seriously. They do have the ability of force to defend themselves.”

The action comes as Donald Trump has spent recent days calling attention to the caravan of Central Americans, many fleeing violence or extortion by gangs and grinding poverty, including thousands of parents with babies or older relatives, slowly making their way by foot into southern Mexico, but still more than 1,000 miles from US soil. Many are aware they are not welcome by the Trump administration but have no idea how their journey is stoking an election cycle political firestorm in the US and inspiring drastic plans from the government.

Where is the migrant caravan from – and what will happen to it at the border? Read more

Trump, who made fear about immigrants a major theme of his 2016 election campaign, has been eager to make it a top issue heading into the 6 November midterm elections, which will determine control of Congress. The president and senior White House officials have long believed the issue is key to turning out his base of supporters.

Late on Thursday, the New York Times and Politico reported that Trump was considering an executive action – aimed at the caravan of people currently travelling through Mexico – to bar migrantsentirely.

The additional troops would provide logistical and other support to the border patrol, said the US official, who spoke on condition of anonymity to discuss a plan that had not been finalized and formally announced.

It’s not unusual for the national guard to help with border security. Active duty troops, however, are rarely deployed within the US except for domestic emergencies like hurricanes or floods.

Trump has used the caravan to bolster his election-season warnings that the US is being infiltrated by illegal immigrants “pouring across the border”, whom he has painted with a sinister brush.

He has claimed, without any apparent basis in fact, that “Middle Easterners” were among the group. At rallies and on Twitter, Trump has tried to portray the Democrats as pro-illegal immigration, even claiming, with no evidence, that Democrats organized the caravan.

He tweeted Thursday: “Democrat inspired laws make it tough for us to stop people at the border” and said he was using the military to respond to what he called a “National Emergency”.

Most of the migrants in the group are Hondurans, seeking to escape poverty and violence, and include families with children.

The caravan swelled to about 7,000 people after crossing the Mexican border on 19 October, but sickness, fear and police harassment have whittled down its numbers.

'God will decide if we make it': Central American caravan presses northward Read more

Trump tweeted a direct message to the migrants Thursday, urging them to return home.

“To those in the Caravan, turnaround [sic],” he wrote. “We are not letting people into the United States illegally. Go back to your Country and if you want, apply for citizenship like millions of others are doing!”

The migrants have largely been disconnected from news reports about them. When asked about Trump’s tweets, they have generally responded that he should stop attacking them and said they would keep heading north.

Candy Guillermo, a 37-year-old migrant in the caravan, said she and the children in group would be considered a threat. “Trump should be more humanitarian,” she said. “We only want to give our kids a better future.”

Migrants who do reach the US border can claim asylum under US and international law. Those who clear an initial screening are often released until their cases are decided in immigration court, which can take several years.

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