WASHINGTON — Undocumented immigrants are increasingly choosing to cross the U.S. border illegally rather than waiting in line to claim asylum at legal ports of entry, according to U.S. Customs and Border Protection data obtained by NBC News.

Immigration lawyers and rights advocates say asylum seekers are opting for illegal crossing because they are growing frustrated with waiting lines caused by Trump administration policies. Advocates say immigrants who might otherwise have presented themselves at legal ports are now going between entry points where, if caught, they can remain in the country while awaiting an asylum hearing.

In recent months, CBP has restricted the number of immigrants who can be processed for asylum at ports of entry and has begun turning back asylum seekers, who must now wait in Mexico while their cases are decided.

CBP data shows that at the same time, the proportion of immigrants caught crossing illegally rather than through legal ports of entry has been rising.

It climbed from 73 percent of border crossings between October 2017 and January to 2018 to 83 percent for the same period ending this January 31. The percentage reporting to legal ports of entry, meanwhile, dropped from 27 percent to 17 percent, even as the overall number of border crossings rose sharply, according to the data.

An official from the Department of Homeland Security, of which CBP is a part, said those abandoning legal entry points may not have legitimate asylum claims.

"The fact that illegitimate asylum seekers may be abandoning efforts at our [ports of entry] means that legitimate asylum seekers at the [ports of entry] can receive protections far more quickly — which has been our goal from the start," said the DHS official. The department declined to comment on the record.

Waiting in Tijuana

In January, U.S. officials finalized a deal with Mexico that forces asylum seekers who present themselves at the legal port of entry in San Diego back across the border to Tijuana. There they must wait months or years, often in unsafe and unsanitary conditions, while an American immigration judge determines whether asylum can be granted. The policy, known as Remain in Mexico, may soon spread to other ports of entry if Mexico agrees to shelter the immigrants at other locations.

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Migrants wait for food at a camp containing hundreds of Central Americans hoping to seek asylum in the U.S. on Dec. 12, 2018, in Tijuana, Mexico. Leah Millis / Reuters

Illegal crossers, meanwhile, do not have to wait in Mexico, even if they are caught. Two DHS officials told NBC News that there are no plans to send asylum seekers back across the border if they are caught crossing illegally, primarily because the Mexican government lacks the infrastructure to shelter them at what are often remote points.

If they are caught and do not claim asylum or pass the initial asylum screening, procedure requires that they are flown back to their home countries. Most current border crossers are not from Mexico but from Honduras, Guatemala and El Salvador.

Immigration advocates and lawyers say immigrants are being warned about the conditions in cities like Tijuana and are increasingly choosing to risk apprehension by the Border Patrol while crossing into the U.S. illegally instead of waiting in Mexico.

Michelle Brané, director of the Women's Refugee Commission, said 9 out of 10 immigrants she spoke to in CBP custody would tell her and her staff they made the choice to cross illegally after other migrants told them the line to enter legally would mean a long wait in a dangerous place.

"They would say, 'There was a line and they told me I would get turned away,' or, 'People told me it's too dangerous and you can't get in that way,'" Brané said.

The most notorious line is in Tijuana, where thousands of immigrants have waited in shelters and tent camps since last fall, hoping to claim asylum to enter San Diego.

The government of Mexico closed a large migrant shelter there in December due to unsanitary conditions and another was shut down last month, forcing families to disperse and often to live on the streets. Also in December, two migrant boys were lured out of their shelter and murdered.

Immigrants in Tijuana keep what is known as "la lista," or the list, to decide who can approach the U.S. border each day to claim asylum, according to affidavits by immigration lawyers. Due to a U.S. policy known as metering, only about 40 to 100 immigrants per day are permitted to enter. CBP is only permitting a maximum of 20 migrants per day to cross into Eagle Pass, Texas, where another group of about 1,800 immigrants has recently arrived.

DHS says metering is a result of only being able to process so many asylum seekers per day, due to limited resources. However, the Trump administration has not increased its manpower for processing asylum claims at the border, though it has increased border enforcement officers and numbers of military troops.

A calculated risk

The number of undocumented immigrants attempting to enter the U.S. from Mexico is not near levels seen in the early 2000s. But the overall number of undocumented immigrants crossing the southern border has risen since a year ago. From October 2017 to January 2018, according to CBP figures, 150,346 crossed the border, a number that surged to 242,667 in the same four-month period ending in January 2019.

The biggest surge has come in the numbers who are crossing illegally. CBP apprehended more than 200,000 crossing illegally between October 2018 and this January, compared to 109,543 a year earlier — nearly doubling the total of illegal crossings.

Working with asylum seekers in Tijuana in December, Kennji Kizuka, senior researcher and policy analyst at Human Rights First, said he saw some immigrants grow frustrated with the wait and try their luck at crossing illegally.

"People were leaving and saying they were about to cross. They had just given up on waiting their turn to get on the list after finding out how long it was and how many months they would be there and how horrible the conditions were," Kizuka said.

But crossing between legal ports of entry also comes with dangers.

Late last year, two children who crossed with their parents died in CBP custody after being picked up in remote areas after making long journeys, where access to water and other basic needs are severely limited.

Border Patrol Chief Carla Provost told Congress on Tuesday that for the first time in U.S. history, families and unaccompanied children make up 60 percent of those arrested between ports of entry. Also, Provost said border patrol is noticing that families and unaccompanied children are traveling in larger numbers: Nearly 68 groups ranging from 100-350 so far in 2019, compared to 13 last year and two the year prior.

Immigration advocates say the large groups are due in part to a "safety in numbers" strategy as families and children are being warned about the dangers not only on the journey but as they await entry to the U.S. in northern Mexico.