President Donald Trump has strongly defended the use of tear gas at the Mexican border to repel a crowd of migrants that included barefoot, crying children as well as angry rock-throwers.

Key points: Donald Trump claims most migrants are dangerous single men, some "stone-cold criminals"

Donald Trump claims most migrants are dangerous single men, some "stone-cold criminals" US authorities arrested 69 people, 39 arrested in Mexico

US authorities arrested 69 people, 39 arrested in Mexico Many migrants feared they might have lost their chance to seek asylum due to the clash

Critics denounced the border agents' action as overkill but the US President kept to a hard line.

"They were being rushed by some very tough people and they used tear gas," he said.

"Here's the bottom line — nobody is coming into our country unless they come in legally."

Mr Trump seemed to acknowledge children were affected but said it was "a very minor form of the tear gas itself" that he assured was "very safe".

"Why is a parent running up into an area where they know the tear gas is forming and it's going to be formed and they were running up with a child?" he asked.

Sorry, this video has expired Border patrol clash with migrants

Without offering evidence, he also claimed that some of the women were not really parents but instead "grabbers" who stole children so they had a better chance of being granted asylum in the US.

The showdown at the San Diego-Tijuana border crossing revealed competing narratives about the caravan of migrants hoping to apply for asylum but stuck in Mexico.

Mr Trump portrays them as a threat to US national security, intent on exploiting America's asylum law, but others insist he is exaggerating to stoke fears and achieve his political goals.

Migrants who have recently arrived at the border are expected to wait months for US authorities to process their claims. ( AP: Ramon Espinosa )

"I think it's so unprecedented that everyone is hanging their own fears and political agendas on the caravan," said Andrew Selee, president of the Migration Policy Institute, a nonpartisan think tank that studies immigration.

"You can call it scary, you can call it hopeful, you can call it a sign of human misery. You can hang whatever angle you want to on it."

Who are the migrants heading towards the US?

Trump rails against migrant caravans as dangerous groups of mostly single men.

That view featured heavily in his speeches during the mid-term election campaign when several were hundreds of kilometres away, travelling on foot.

Officials said about 500 members are criminals but did not back that claim up.

Mr Trump tweeted the caravan at the border included "stone-cold criminals".

At the sports complex where most of the migrants in the caravan are staying, there are 4,938 people there.

That includes 933 women, 889 children and 3,105 men, Tijuana's social services department director Mario Figueroa said.

Mexico's foreign ministry has asked the US for a full investigation into the use of tear gas. ( AP: Pedro Acosta )

About 300 troops who had been deployed in south Texas and Arizona as part of a border security mission were moved to California.

The military's role is limited largely to erecting barriers along the border and providing transportation and logistical support to Customs and Border Protection.

Democrats and immigrant rights groups blasted the border agents' tactics.

"These children are barefoot. In diapers. Choking on tear gas," California Governor elect Gavin Newsom tweeted.

"Women and children who left their lives behind — seeking peace and asylum — were met with violence and fear. That's not my America."

Donald Trump says the migrants are a threat to national security. ( AP: Greg Bull )

The day after the incident, Mexico's foreign ministry asked the US for "a full investigation".

US Customs and Border Protection Commissioner Kevin McAleenan said the administration's concerns about the caravan "were borne out and on full display" on Sunday.

Mr McAleenan said hundreds — perhaps more than 1,000 — people attempted to rush vehicle lanes at the San Ysidro crossing.

Mexican authorities estimated the crowd at 500.

The chaos followed what began as a peaceful march to appeal for the US to speed processing of asylum claims.

Donald Trump questioned why parents were running into areas where tear gas had been released. ( AP: Ramon Espinosa )

ABC/wires