Homeland Security Secretary Kirstjen Nielsen announced that President Donald Trump has ordered National Guard personnel to the Mexican border

President Donald Trump has ordered National Guard personnel to the southern US border, his administration said Wednesday, in a bid to clamp down on illegal immigration from Mexico.

"We continue to see unacceptable levels of illegal drugs, dangerous gang activity, transnational criminal organizations and illegal immigration flow across our southern border," secretary of Homeland Security Kirstjen Nielsen told reporters at the White House.

"This threatens not only the safety of our communities and children but also our rule of law. It's time to act."

"The Department of Defense and the Department of Homeland Security have been directed to work together with our governors to deploy the National Guard to our southwest border, to assist the border patrol," she said.

"We do hope that the deployment begins immediately," she added. "Today is the day we want to start this process. The threat is real."

The US National Guard has previously been deployed to help patrol the southern border, in 2010 under former president Barack Obama, and from 2006-2008 under George W. Bush.

Trump has ratcheted up the pressure on both Congress and America's southern neighbor Mexico in recent days to take action to stem illegal immigration.

The president has been infuriated by reports of a caravan of Central American migrants trekking towards the US border, threatening to axe the North American Free Trade Agreement if Mexico did not stop them.

"Until we can have a wall and proper security, we're going to be guarding our border with the military," Trump told journalists on Tuesday.

The caravan's leaders on Wednesday said they were scrapping their plans to cross into the United States.