GRANDE-SYNTHE, France—On a rainy morning, Leila Xider, her husband and their 2-year-old son left the Jungle—a sprawling migrant camp in Calais—to attempt the journey to the U.K. from a nearby town.

Instead, the Iraqi family is now holed up 30 miles north on a swampy patch of land at the edge of Grande-Synthe, a suburb of Dunkirk, where another camp is rapidly growing.

"This is worse than the Jungle," said the 25-year-old Ms. Xider. But her husband, she says, won't give up. "We've spent all we had to get here."

Camps such as the one in Grande-Synthe are burgeoning along France's northern coast, as well as parts of Belgium, as a heavy police presence in the main port city of Calais forces migrants to seek a different route to reach the U.K. Once there, many believe they will find better economic conditions to start a new life.

Police in the nearby ports of Dunkirk, Le Havre, Dieppe and Belgium's Zeebrugge say they are facing an unprecedented influx of migrants, who every night try to sneak onto trucks headed for the U.K. While authorities routinely dismantle some of the camps, others, like the one in Grande-Synthe, have become a more permanent fixture.

"They try a few times in Calais and if they can't make it, they try in other ports along the coast," said Frank Demeester, a spokesman for the prosecutor's office in the Belgian town of Bruges.

The situation in Grande-Synthe and other towns is casting a harsh light on France's failure to find a solution to the migrant crisis in the north of the country. While it has successfully sealed off its border with the U.K. in Calais—building high razor-wire fences around the entrance to the Channel tunnel and the port, and deploying over 1,000 police—the government has done little to prevent the crisis from spreading to neighboring towns.

"We shouldn't simply move the problem but solve it," said Bruno Lafosse, chief of staff to the mayor of Dieppe, 100 miles south of Calais. In December, about 30 migrants stormed the ferry terminal, leaving local police overwhelmed, a first for the quiet port town, said Mr. Lafosse.

Dunkirk border police says they catch close to 500 migrants trying to get to the U.K. by stowing away in trucks every week, compared with just a handful six months ago. It is unclear how many make it to the U.K. undetected.

In the port of Le Havre, 170 miles down the coast, where a few months ago, weeks could go by without police spotting any migrants in the port, about a dozen people hiding in the back of trucks are caught every week, say officials.

"Things here are taking a new turn," said Franç ois Lobit, a county police deputy chief.

So far, French and British authorities have focused most of their interactions with migrants on emptying out the Jungle, the camp in Calais that sprawls across windswept dunes on the channel's French coast, relocating thousands of people to other parts of France and Europe.

British authorities continue to work closely with the French to bolster ports in northern France, a spokeswoman for the U.K. Home Office said. "The message is clear—those in genuine need of protection will find it elsewhere in France," she said.

French immigration officers have started visiting the Grande-Synthe camp to persuade refugees to apply for asylum. According to officials, about 500 migrants who have been camped out there have been transferred to shelters since October. But many migrants are unwilling to give up on their goal of reaching the U.K.

After three months in Calais, and numerous failed attempts to cross the Channel, Ms. Xiler and her family decided to relocate to Grande-Synthe.

"We have been stuck in hell here for a month," she said.

Police reinforcements have been sent to the camp, but unlike in Calais, the government has done nothing to improve living conditions, alarming local officials and aid organizations.

Grande-Synthe Mayor Damien Carê me, initially allowed migrants to stay in his town, setting up basic amenities for them to use.

Until last summer, fewer than 50 migrants camped in a clearing in the woods just outside the city, and mostly went unnoticed. But late last year, as hundreds of migrants—many of them refugees fleeing war in Iraq and Syria—started arriving every week, the small settlement grew into a ramshackle tent village. As winter came, rivers of mud started flowing through the camp, trash piled up, and disease spread.

Mr. Carê me turned to the government for help.

"They told me they won't give me a penny," he said.

The widespread presence of people smugglers, who charge several thousand euros to sneak migrants past border controls, discouraged French officials from taking any steps to improve living conditions at the camp.

"We aren't going to set up new facilities so smugglers can charge migrants to use them," a senior French government official said.

Mr. Carê me is now working with the humanitarian organization Doctors Without Borders to relocate the camp to another part of town. The group, which runs a small clinic in Grande-Synthe, agreed to pay €2 million ($2.24 million) to set up the new camp, while the city will pay another €400,000.

"We're basically doing the government's job," said Michel Janssens, mission head at Doctors Without Borders.

Within a few weeks, the migrants should be transferred to the new, temporary camp, wedged between the highway and the railways, where heated tents and basic amenities will be set up. Doctors Without Borders has started preparing the camp's residents—both migrants and smugglers—for the move.

But police won't allow more than 2,500 inside the new settlement. "I don't want another Calais," said Mr. Carê me.

Jenny Gross in London contributed to this article.

Write to Noemie Bisserbe at noemie.bisserbe@wsj.com

(END) Dow Jones Newswires

February 07, 2016 20:15 ET (01:15 GMT)