NEW YORK (Reuters) - U.S. border officers apprehended more than 132,000 people crossing from Mexico in May, an increase over the previous month and the highest monthly level since 2006, reaching what U.S. officials said on Wednesday were “crisis” levels.

Workers weld sections of a newly replaced border wall with Tijuana, Mexico near the the Otay Mesa border crossing in San Diego, California May 31, 2019. REUTERS/Mike Blake

U.S. President Donald Trump is frustrated with the rising number of mostly Central American migrants crossing into the United States and the new arrest numbers coincided with his threat to impose across-the-board tariffs on Mexican goods if Mexico does not do more to stem the flow of people.

“Our nation is experiencing an unprecedented border security and humanitarian crisis on the southwest border, both at and between our ports of entry,” Randy Howe, the executive director for operations, Office of Field Operations at U.S. Customs and Border Protection (CBP), told reporters on a call.

He said on June 4 alone, CBP apprehended more than 4,100 people and had 19,293 people in custody.

“We are bursting at the seams. It is unsustainable,” Howe said.

Trump made reducing illegal immigration one of his main platforms in the 2016 election campaign but he has had limited success in carrying out aspects of his policy as president.

Border agents apprehended 132,887 people on the southwest border in May, according to CBP data, a more than 30 percent increase from April. The last time apprehensions topped that level was more than a decade ago in March 2006, the data shows.

Overall arrests were still lower than at other peak periods of illegal immigration since the 1970s. But U.S. officials say they are struggling to handle a demographic shift in the type of migrants now crossing: from mainly single, adult Mexicans trying to cross clandestinely who could be turned around, to mostly families from Guatemala, El Salvador and Honduras seeking asylum and who cannot be quickly deported under U.S. law.

More than 63% of those apprehended in May were children and people traveling as families.

Border officials deemed a further 11,391 people “inadmissible” in May, up 12% from the previous month. That category includes people who present themselves for legal admission at a port of entry but are turned away, and people who seek asylum at ports of entry.

To deal with the influx, 731 CBP officers have been redeployed from airports, seaports and legal ports of entry to assist with the processing of migrant families, which is increasing traveler and trade wait times, Howe said.

Border agents have been limiting the number of migrants they process at legal ports of entry, which immigration advocates say has pushed more people to try to cross illegally and turn themselves in.

FAMILIES RELEASED

Most families are released to pursue their asylum claims in U.S. immigration courts because of legal limits on how long children can be held in detention. Pursuit of such claims can take years due to ballooning court backlogs.

Since March 19 this year, over 75,000 non-criminal family units were released by Border Patrol, acting CBP Commissioner John Sanders said. “We began doing that because our facilities were over capacity.”

“As word of mouth and social media continues to spread about the news of a quick release into the United States, more migrants are emboldened to make this dangerous journey,” Sanders said on the call.

U.S. officials say they do not have the proper facilities to handle the more than 55,000 children who passed through Border Patrol custody in May. A U.S. government watchdog reported “dangerous” overcrowding at one border patrol facility in El Paso, Texas. And at least six minors have died while they were in U.S. custody or shortly after being released within the past year.

Border Patrol said it had rescued more than 3,000 people so far in the 2019 fiscal year, which began last October, including a “massive increase” in the number rescues of migrants crossing the Rio Grande river.

Migrants are increasingly trying to cross in large groups - 182 such groups have been caught so far this fiscal year compared to 13 in 2018 and just 2 in 2017. On May 30 in Del Rio, Texas, 117 migrants from Central and South Africa, were apprehended.

Just before Trump threatened tariffs on Mexico last week, he tweeted a May 29 Border Patrol video showing the apprehension of more than 1,000 migrants, mostly children and families.