TIJUANA, Mexico - US border agents fired tear gas on hundreds of migrants protesting near the border with Mexico on Sunday after some of them attempted to get through the fencing and wire separating the two countries.

Authorities also shut down all traffic at the busy crossing between San Diego, California, and Mexico's Tijuana, where thousands are waiting to apply for asylum.

The situation devolved after the group began a peaceful march to appeal for the United States to speed processing of asylum claims for Central American migrants marooned in the Mexican city.

The dramatic shutdown took place only three days after US President Donald Trump threatened to close the "whole border" with Mexico if "it gets to a level where we're going to lose control or people are going to start getting hurt".

Mexican police had kept the migrants from walking over a bridge leading to the Mexican port of entry, but they pushed past officers to walk across the Tijuana River below the bridge. More police carrying plastic riot shields were on the other side, but migrants walked along the river to an area where only an earthen levee and concertina wire separated them from US Border Patrol agents.

Some saw an opportunity to breach the crossing.

A reporter saw US agents shoot several rounds of tear gas after some migrants attempted to penetrate several points along the border. Mexico's Milenio TV showed images of migrants climbing over fences and peeling back metal sheeting to enter.

Honduran Ana Zuniga, 23, also said she saw migrants opening a small hole in concertina wire at a gap on the Mexican side of a levee, at which point US agents fired tear gas at them.

Children screamed and coughed. Fumes were carried by the wind toward people who were hundreds of feet away.

"We ran, but when you run the gas asphyxiates you more," Zuniga said while cradling her 3-year-old daughter Valery in her arms.

Mexico's Interior Ministry said around 500 migrants tried to "violently" enter the US. The ministry said it would immediately deport those people and would reinforce security.

Throughout the day, US Customs and Border Protection helicopters flew overhead, while US agents kept vigil on foot beyond the wire fence in California. The Border Patrol office in San Diego said via Twitter that pedestrian crossings were suspended at the San Ysidro port of entry at both the East and West facilities. Every day more than 100,000 people enter the US there.

US Homeland Security Secretary Kirstjen Nielsen said the authorities will continue to have a "robust" presence along the Southwest border and they will prosecute anyone who damages federal property or violates US sovereignty.

AP - Xinhua - AFP