Trump to send National Guard troops to US-Mexico border, Mexico miffed

US President Donald Trump has signed a proclamation to send the National Guard to the country's border with Mexico, a senior White House official has said.

Homeland Security Secretary Kirstjen Nielsen said on Wednesday at the White House press briefing that the signing would be done in conjunction with governors and that the administration hoped the deployment would begin "immediately", Fox News 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," she said.

"The President has directed that the Department of Defense and the Department of Homeland Security work together with our governors to deploy our National Guard to our southwest border to assist the border patrol," she said. "The president will be signing a proclamation to that effect today."

Details about what the National Guard would do and how many would be deployed and for how long were not immediately disclosed.

Under the George W. Bush administration, deploying the National Guard to the border cost $415 million.

Nielsen pointed to what she described as increasing fraud and exploited loopholes among arrivals on the southern border, saying traffickers have been advertising that if migrants have children with them, then they are more likely to be released into the US. She also said that almost 50 per cent of arriving aliens are from Central America.

"Traffickers and smugglers know that these individuals cannot under US law be easily removed in an expeditious way back to their country of origin and so they exploit the loophole," she said, adding that the ability to game the system acts as a magnet for more migrants.

She said that the administration has drafted legislation and will ask Congress to provide legal authority and resources to address the problem.

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

Trump had tweeted earlier on Wednesday that he would "be taking strong action today (Wednesday)" on the Mexico border, a day after he said that he wants to send the military to secure it until a wall is built.

Mexico criticises move

The Mexican Senate has "categorically" rejected the US initiative to militarise the US-Mexico border to thwart undocumented migrants.

The response came hours after US President Donald Trump on Wednesday signed a decree to immediately deploy the National Guard to patrol the 3,200-km border the two countries share, Xinhua news agency reported.

Deploying army troops or the National Guard in the region would be "yet another wrong" against the Mexican community, Mexican lawmakers said in a letter addressed to their Interior Ministry.

The letter, signed by 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".

Trump has stepped up his anti-immigration rhetoric recently. He told reporters on Tuesday: "Until we can have a wall and proper security, we're going to be guarding our border with the military."