Pence blasts 'weak' immigration laws in speech at US-Mexico border

US Vice President Mike Pence blasted "weak" and "ineffective" immigration laws today during a trip to the border with Mexico, as a caravan of immigrants two hours to the west prepared to claim asylum in America.

Pence's remarks came as he toured a construction site for a barrier in the Californian border town of Calexico, 200 kilometres east of the San Diego-Tijuana crossing where the group of at least 150 people has amassed.

"I stand before you today when most of the attention of the nation has been focused on the so-called caravan," Pence said.

"As the president said on Saturday night, this situation is a direct result of our weak immigration laws and our porous border." The visit came after the Central American migrants arrived Sunday in the border town of Tijuana, part of a caravan of more than 1,000 people who set out from southern Mexico in late March.

Pence described them as "victims" of activists, smugglers and the media and called on Congress to close "dangerous loopholes" allowing people to claim asylum in the US rather than the first safe country they enter.

Media coverage of the US-bound caravan has triggered a flurry of furious tweets from Trump, who ordered thousands of National Guard troops to the border and called on Mexico to stop the migrants.

"We don't have borders, we don't have a country. And I've been watching for weeks as the caravan came up," Trump told reporters at the White House today.

"And, you know, the Mexican laws are very tough on immigration, extremely tough." He reiterated his call for an end to the "catch and release" protocol under which people detained by US authorities as unlawful immigrants can be released while they wait for a hearing.

"You've got to have a wall. And we need border security, and we will have border security. As you know, we're sending many, many National Guardsmen down to the border," Trump said.

"Most of them are already there. And that's having a big impact. But we need a change in the law. Catch and release is ridiculous." Pence's visit comes four weeks after Trump toured wall prototypes in Otay Mesa, just north of the border.

Plans were launched in 2009 for the Calexico structure -- which replaces around two miles of decades-old barriers according to the Los Angeles Times -- although Trump sowed confusion last month when he tweeted photos and wrote, "Great briefing this afternoon on the start of our Southern Border WALL!"