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Thousands fleeing conflict or poverty in Nigeria, Cameroon, Bangladesh, Haiti and Cuba have traveled across oceans, through the jungles and mountains of South America, up through Central America, on a route that — so far — ends here: the steamy, crumbling Mexican city of Tapachula, near the Guatemala border.

Over 1,500 of them while away the weeks — or months— in a park dotted by giant ceiba trees and vines, awaiting exit visas that never seem to come, like a Mexican version of the movie "Casablanca." Some say they've given up hope of reaching the United States and just want papers that will allow them to work in Mexico — but northern Mexico, where wages are higher. The government is not prepared to grant that, so it keeps them here, waiting. Perhaps for an asylum ruling, perhaps residency status.

Their lives are a daily round of boredom, a lack of answers from authorities, dirty, overcrowded bathrooms and insufficient food. Those who still have money sometimes sneak out of the compound by jumping a wall and buying their own groceries to cook over open fires. The international melange of migrants seems to share a taste for rice and lentils, not the tortillas, beans and eggs provided by Mexican authorities.

Maureen Meyer, director for Mexico and migrant rights at the Washington Office on Latin America, said that word quickly spread through international smuggling networks that Mexico had become more permissive for migrants. Attention drawn to the large caravans meandering north to the U.S. last year, combined with Mexico's fast-track for thousands of humanitarian visas in January, appeared like welcome mats on the global stage. At the same time, it became more difficult for migrants in Asia or Africa to reach Europe.

Now the Mexican government is trying to get a better handle on the flows -- and perhaps even limit transit visas -- amid pressure from U.S. President Donald Trump to clamp down on migration to the U.S.

"They didn't fully assess the messaging of being a more open country," said Meyer. "It's obviously a very difficult situation" for Mexico.

The backlog on Mexico's southern border also appears, in part, to be a function of budget cuts, as well as the country's limited capacity to handle large numbers of migrants, especially those from distant countries, some of which lack the infrastructure to handle repatriations.

Trump has repeatedly threatened to close the U.S. border with Mexico if the heavy flow of migrants to the U.S. continues.

"The Mexican government's decision to detain as many migrants as possible, after President Trump put pressure on them to do so, has made it clear just how many third-country nationals from outside Central America are actually in the country," said Andrew Selee, president of the Washington-based Migration Policy Institute.

Many of the migrants waylaid in southern Mexico take the interminable wait for visas in stride; they have been through much worse on their long torturous journeys.

"The thief took my backpack, with my telephone, my documents, my passport. He took everything," recounted Paul Eneceron, a 21 year-old economy student from Port-au-Prince, Haiti, who set out on Jan. 13 from Chile — where he worked packing fish and baking cakes for 2 1/2 years — hoping to reach Mexico, where he has relatives among the thousands of Haitians who have settled in Tijuana.

He crossed through Peru, Ecuador and Colombia, but like most, he found the border between Panama and Colombia to be the most dangerous. It is patrolled by bandits toting guns and machetes who call themselves "Los Indios" ("The Indians") even though they have no links to Panama's indigenous population. It was there that the bandits sprang out of the jungle. "He (the robber) pointed the pistol at me and said: 'Hand over everything.' And I gave him my backpack."

One of the longest routes was that traveled by Musa Kolo, a welder from Nigeria. He said he fled violence from the Islamic extremist group Boko Haram in Nigeria's Borno State several months ago and made his way to the Ivory Coast, where he stowed away on a freighter. Once he was discovered, the crew took pity on him and left him off in Brazil, and he made his way up through Colombia and on to Panama. From there the route — now well-worn — leads through Costa Rica, Nicaragua, Honduras, Guatemala and finally Mexico.

Like Eneceron, Kolo struggled in the roadless Colombia-Panama border area known as the Darien Gap.

In previous months, some migrants had been lucky enough to pay to make the passage on small boats that avoided much of the jungle, and the robbers. But in February one of the boats sank, killing about 19 migrants, and the service was shut down. So migrants like Kolo were left to hike. "I spent nine days in the jungle, walking in the jungle. We had no water, our food finished after about three days because we didn't bring a lot of food. We just kept on walking."

"Now, I just spend my time praying they will give me my papers," said Kolo, who said he would consider staying to work in Mexico.

The large numbers of transcontinental migrants traversing that dangerous route speaks to the desperation of their situations at home, said Meyer, of the Washington Office on Latin America

Charles Lwanga, a 38-year-old teacher, said he fled Cameroon two months ago to escape violence against the English-speaking population by the Francophone majority-government there. Lwanga travelled to Ecuador and then headed north, hoping to seek asylum in the United States.

At the Panama-Colombia border, Lwanga was lucky. "My group wasn't robbed," though others were, and he said, "Some people just died out of exhaustion ... We saw bodies, fresh bodies, and skeletons of people who died some time before."

Now, like so many others, after covering so many miles, his trail seems to have ended in Tapachula, where he said immigration authorities endlessly put off any response.

"These are the most arrogant immigration authorities I have seen on this journey," Lwanga said. "Every minute they threaten you, to take you to another camp which is worse than this."

Sometimes migrants travel a staggeringly long way to go just a short distance. Cuba is 90 miles from Florida, but Alain Romero, a baker and desert chef from Havana, has travelled nine months and thousands of miles only to be stalled in southern Mexico.

Romero, who hopes to get a job in the United States and send money back to his wife and two daughters, flew from Cuba to French Guayana, then made his way through Brazil and Colombia to follow the Panama route north, suffering the same brushes with bandits the others did.

Now he's been waiting 27 days at another of the camps that Mexico has set up to house — and, seemingly delay with endless paperwork — another group of hundreds of migrants in the town of Mapastepec, to the west of Tapachula.

Though the days grind by with no answer as to when his papers will come, Romero is willing to wait. "Things are worse in Cuba," he said. "We are willing to keep waiting. There are people who got here ahead of us; we have to wait our turn."

In the past, Mexico swiftly issued exit visas to Cuban nationals so that they could move through the country and toward the U.S.

But Mexican President Andrés Manuel López Obrador's promises of a new, more humane approach to migration seem to be melting — under U.S. pressure — into the old, deportation-oriented policies of his predecessor, Enrique Peña Nieto.

Mexico has deported tens of thousands of migrants in recent months and officials are now being more selective about who gets humanitarian visas.

For some migrants, the wait in southern Mexico is likely to be so long their immigration status will change by itself, through the work of nature.

Marc Louis Rosetanie, 26, nine months pregnant, and her husband Marc Roselin, 29, from Cap Haitien, Haiti, arrived just a few days ago in Mexico over the same torturous Central American route. Roselin stood with his wife outside an immigration detention center in Tapachula, wondering who he should ask about applying for a humanitarian visa — or obstetrical care for his wife.

"Soon, we will have a Mexican child," Roselin said. "That may change things."

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Associated Press writer Amy Guthrie contributed to this report from Mexico City.