US President Donald Trump's administration has denied reports that it is considering a proposal to mobilise as many as 100,000 National Guard troops to crack down on undocumented immigrants, including millions living far from the Mexico border.

A draft memo written by John Kelly, US homeland security secretary, which was obtained by the Associated Press and released by the news agency on Friday, called for the unprecedented militarisation of immigration enforcement as far north as Portland, Oregon, and as far east as New Orleans, Louisiana.

The report was revealed shortly after Trump launched a campaign that has resulted in hundreds of undocumented migrants being detained by authorities across the country.

In the leaked proposal, four states that border on Mexico included California, Arizona, New Mexico and Texas - but it also encompasses seven states contiguous to those four - Oregon, Nevada, Utah, Colorado, Oklahoma, Arkansas and Louisiana.

Report '100 percent false'

For his part, Sean Spicer, White House spokesman, said the AP report was "100 percent false" and "irresponsible".

''There is no effort at all to utilise the National Guard to round up unauthorised immigrants," he said.

Governors in the 11 states would have a choice whether to have their guard troops participate, according to the memo.

While National Guard personnel have been used to assist with immigration-related missions on the US-Mexico border before, they have never been used as broadly or as far north.

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The memo is addressed to the then-acting heads of US Immigration and Customs Enforcement and US Customs and Border Protection.

It would serve as guidance to implement the wide-ranging executive order on immigration and border security that President Donald Trump signed on January 25. Such memos are routinely issued to supplement executive orders.

Also dated January 25, the draft memo says participating troops would be authorised "to perform the functions of an immigration officer in relation to the investigation, apprehension and detention of aliens in the United States".

It describes how the troops would be activated under a revived state-federal partnership programme, and states that personnel would be authorised to conduct searches and identify and arrest any unauthorised immigrants.

AP said its requests to the White House and the Department of Homeland Security for comment and a status report on the proposal were not answered.

The draft document has circulated among DHS staff over the last two weeks. As recently as Friday, staffers in several different offices reported discussions were under way.

If implemented, the impact could be significant. Nearly one-half of the 11.1 million people residing in the US without authorisation live in the 11 states, according to Pew Research Center estimates based on 2014 Census data.

Use of National Guard troops would greatly increase the number of immigrants targeted in one of Trump's executive orders last month, which expanded the definition of who could be considered a criminal and, therefore, a potential target for deportation.

That order also allows immigration agents to prioritise removing anyone who has "committed acts that constitute a chargeable criminal offence".

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Under current rules, even if the proposal is implemented, there would not be immediate mass deportations. Those with existing deportation orders could be sent back to their countries of origin without additional court proceedings. But deportation orders generally would be needed for most other unauthorised immigrants.

The troops would not be nationalised, remaining under state control.

The 287(g) programme, which Trump included in his immigration executive order, gives local police, sheriff's deputies and state troopers the authority to assist in the detection of immigrants who are in the US illegally as a regular part of their law enforcement duties on the streets and in jails.

The draft memo also mentions other items included in Trump's executive order, including the hiring of an additional 5,000 border agents, which needs financing from Congress, and his campaign promise to build a wall between the US and Mexico.

Allowing guard troops to operate inside non-border states also would go far beyond past deployments.

In addition to responding to natural or man-made disasters or for military protection of the population and critical infrastructure, state guard forces have been used to assist with immigration-related tasks on the US-Mexico border, including the construction of fences.

Last week, ICE officers arrested more than 680 people around the country in what Kelly said were routine, targeted operations; advocates called the actions stepped-up enforcement under Trump.