TIJUANA, Mexico - Central American migrants stuck on the threshold of the United States in Mexico breached the border fence on Monday, risking almost certain detention by US authorities but hoping the illegal entry will allow them to apply for asylum.

Since mid-October, thousands of Central Americans, mostly from Honduras, have traveled north through Mexico toward the US in a caravan, some walking much of the long trek.

US President Donald Trump has vowed to stop the migrants entering, sending troops to reinforce the border and attempting a procedural change, so far denied by the courts, to require asylum-seekers to remain in Mexico while their cases are heard.

Frustrated and exhausted after weeks of uncertainty, many of the migrants have become desperate since getting stuck in squalid camps in the Mexican border city of Tijuana.

So a number opted to eschew legal procedures and attempt an illegal entry from Tijuana as dusk fell on Monday at a spot about 450 meters away from the Pacific Ocean.

In less than an hour, Reuters reporters observed roughly two dozen people climb the approximately 3-meter fence made of thick sheets and pillars of metal. They chose a place in a large overgrown ditch where the fence is slightly lower.

Just before dusk, three thin people squeezed through the fence on the beach and were quickly picked up by the US Border Patrol, witnesses said.

But along the border inland as darkness descended, more and more migrants followed, many bringing children.

Some used a blanket as a rope to help loved ones get over.

A mother and her children made it over the first fence and disappeared into the night.

The sight of them climbing the fence encouraged others, even as a helicopter patrolled overhead on the US side.

Earlier, Karen Mayeni, a 29-year-old Honduran, sized up the fence while clinging to her three children, aged six, 11 and 12.

"We're just observing, waiting to see what happens," Mayeni said. "We'll figure out what to do in a couple of days."

Ninety minutes later, she and her family were over the fence.

A number of migrants ran to try to escape capture, but most of them walked slowly to where US Border Patrol officials were waiting under floodlights to hand themselves in.

Some of the migrants are likely to be economic refugees without a strong asylum claim, but others tell stories of receiving politically motivated death threats in a region troubled by decades of instability and violence.

Applying for asylum at a US land border can take months, so if migrants enter illegally and present themselves to authorities, their cases could be heard quicker.

US officials have restricted applications through the Chaparral gate in Tijuana to between 40 and 100 per day.

Reuters