Washington, Apr 5 (IANS): US President Donald Trump has officially signed a memorandum to deploy the National Guard to the country's border with Mexico, a move that followed days of him calling for a larger crackdown on illegal immigration.

In the memo signed on Wednesday evening, Trump declared that Defence Secretary James Mattis "shall request use of National Guard personnel" to secure the border and that Homeland Security Secretary Kirstjen Nielsen will work with him to train and direct the troops in that mission.

Key details like the numbers of troops, how long they will be deployed, how much it will cost and where they will go are still being finalised, CNN reported.

"Despite a number of steps this administration has taken... we continue to see unacceptable levels of illegal drugs, dangerous gang activity, transnational criminal organisations and illegal immigration flow across our border," Nielsen said, adding that troops could be sent to the border "immediately".

The move followed days of public fuming by Trump about immigration policy, during which he tweeted about immigration legislation in Congress, a caravan of migrants making its way through Mexico and what he called "weak border laws".

He tweeted earlier on Wednesday that he would "be taking strong action" on the Mexico border, a day after he said that he wants to send the military to secure it until a wall was built.

Nielsen cited a historic uptick in border crossings in spring months like April and advertising from smuggling groups as reasons for the move, calling it in part "anticipating".

"Why not yesterday and tomorrow? Today is the day we want to start this process," she said. "The threat is real."

"We will not allow illegal immigration levels to become the norm... More than 1,000 people a day, 300,000 a year violating our sovereignty as a nation will never be acceptable to this President," she said.

The memo instructed the two secretaries and Attorney General Jeff Sessions to put together a report within 30 days on further steps the administration can take to combat what it called a "crisis" and "a drastic surge of illegal activity on the southern border".

Meanwhile, the Mexican Senate "categorically" rejected the US move and said that deploying Army troops or the National Guard in the region would be "yet another wrong" against the Mexican community.

The letter, signed by Mexican Senate President Ernesto Cordero, called on the Mexican government to suspend bilateral cooperation in immigration matters and the fight against organised crime until "Trump behaves with civility and shows respect to Mexico".