TIJUANA, Mexico: Thousands of migrants have crossed into the United States in recent days, from California to Texas, with many more still arriving by bus and cargo trains to Mexican border towns on the heels of record migration flows further south.

The dramatic increase along the border - notably in San Diego, California, and the Texan cities of El Paso and Eagle Pass - marks a turning point after numbers had plummeted in recent months, and could create fresh political challenges for U.S. President Joe Biden heading into election season.

Biden in May rolled out a new policy to deter illegal crossings, including deporting migrants and banning re-entry for five years, as his administration grappled with migration at record highs.

Within a month the tougher measures drove the border-crossing rate down some 70%.

But a recent uptick in arrivals at the border, combined with vastly higher numbers of people on their way north across Central and South America and riding dangerous cargo trains through Mexico, suggest the early deterrent effect is wearing off.

Experts say the U.S. lacks the capacity to detain and process migrants at the border, often making it impossible for the administration to carry out the harsh penalties it announced in May.

As a result, some asylum seekers who cross illegally are being released into the U.S. with a future court date, rather than being deported - becoming success stories repeated back to migrants still en route.

"The (Biden administration) hit on a smart strategy, but they don't have the resources or capacity to implement it," said Andrew Selee, head of the Migration Policy Institute.

In response to questions from Reuters, U.S. Customs and Border Protection (CBP) said it was "safely and efficiently" processing migrants, and would impose consequences, including deportation, on migrants without a legal basis to stay in the country.

Mexican President Andres Manuel Lopez Obrador on Thursday derided the lack of an international plan to help countries lift their citizens out of poverty and thus avoid a key migration driver. He praised Biden for creating legal pathways for migrants but said they needed to be expanded.

'WE HAVE TO DO SOMETHING'

In Tijuana on Wednesday evening, on the other side of the border from San Diego, several dozen people prepared to spend the night sleeping on the ground at a border entry point ahead of appointments early the next day, secured through a mobile app called CBP One, to enter the U.S. and request asylum.

But not everyone wants to wait.

"My wife's family, and other people who came to Mexico with us, say they crossed (without an appointment) and nothing happened," said Venezuelan migrant Oscar Suarez, 27, sitting in a Tijuana plaza near the border with his pregnant wife, 2-year-old son and two brothers.

He said he preferred to try the same strategy rather than wait on CBP One to obtain an appointment. Demand for appointments far outweighs the 1,450 time slots available borderwide per day, and Suarez said he worried that his family would not survive a long wait.

"Our money ran out, and we don't have anything to eat," he said. "All the shelters in Tijuana are full. We have to do something."

Enrique Lucero, Tijuana's director of migrant affairs, said migration slowed after the U.S. policy change in May, but over the last several weeks has been picking up. Officials have tallied 65 nationalities of people in the city, he said.

Hundreds of migrants who crossed without appointments have been forced to wait between two border walls.

Within the last eight days, CBP had processed more than 5,000 migrants in the San Diego area, a San Diego official said on Thursday.

In Ciudad Juarez, opposite El Paso, hundreds of migrants squeezed past barbed wire to cross the Rio Grande river into the U.S., forming a line next to the border while awaiting processing by U.S. officials.

CBP has logged more than 1,000 migrant encounters daily in the El Paso area in the last several days, according to data published by the city of El Paso.

Migrants are also crossing the river at the Texas city of Eagle Pass, where officials signed an emergency declaration on Tuesday to seek funding for additional services, and railroad operator Union Pacific said it was forced to shut service to Mexico.

Groups of migrants have been as large as 1,000 or 2,000 people, including several hundred migrants who braved a hailstorm to wade through the river.

Mexican railroad operator Ferromex this week suspended service on 60 trains to discourage migrants, who perilously ride north on cargo wagons.

LONG JOURNEYS

A record of about 82,000 people last month entered Panama overland from South America, according to the International Organization for Migration (IOM), crossing the dangerous Darien Gap jungle that has transformed in recent years from a nearly impassable barrier to a migration thoroughfare.

As many as half a million people could end up crossing by year-end, double the number of 2022, said Giuseppe Loprete, head of IOM in Panama.

Most people crossing the Darien Gap left their home countries due to lack of employment, according to a July U.N. survey.

An unprecedented number of migrants entering Mexico hail from othercontinents, as the trek to the U.S. southern border increasingly becomes a global migration route.

"Unfortunately, the picture is that many countries are becoming countries of expulsion," said Giovanni Lepri, representative of the U.N. refugee agency in Mexico.

He said violence, economic distress and the growing impacts of climate change were driving mass displacement across the Americas and beyond.

The number of African migrants registered by Mexican authorities so far this year is already three times as high as during all 2022.

"It's a structural, deeper problem. There's an exacerbated crisis globally, in many countries. People don't leave their countries because they want to - they do it out of need," Lopez Obrador told reporters on Thursday.