U.S. Customs and Border Patrol agents and K-9 security dog keep watch at a checkpoint station, on Feb. 22, 2013, in Falfurrias, Texas. (Eric Gay/AP)

With the Border Patrol distracted by a surge of Central American migrants crossing into south Texas, Mexican cartels have had an easier time smuggling illegal drugs across the border, according to agents and state officials here.

The arrival of large groups of women and children on the U.S. side of the Rio Grande is pulling agents away from their patrol stations elsewhere along the border, creating gaps in coverage that the traffickers can exploit, according to Chris Cabrera, the Border Patrol union representative here.

The smugglers wait on the southern banks of the Rio Grande as migrant groups as large as 250 wade across at dusk and turn themselves in to the Border Patrol, he said. Then groups of single men proceed to cross under cover of darkness, hoping to slip through.

“After that they send over the dope,” Cabrera said, with U.S. officers too busy to stop it.

Texas Attorney General Greg Abbott (R) and Gov. Rick Perry (R) have echoed the complaints. In a letter this month to Department of Homeland Security Secretary Jeh Johnson, Abbott asked for $30 million for law enforcement officers to fill in the gaps because “we have grave concerns that dangerous cartel activity, including narcotics smuggling and human trafficking, will go unchecked because Border Patrol resources are stretched too thin.”

The most recent statistics from the U.S. Drug Enforcement Administration (DEA) show that narcotics seizures have fallen across the entire border with Mexico this year, with the drop being larger in Texas than the average.

In Texas, combined seizures of marijuana, cocaine, heroin and methamphetamine fell by 34 percent — from 374,812 to 246,976 kilograms — between Jan. 1 and June 14 compared with the same period last year. Seizures in Arizona and California fell by 26 percent in each state. The decline was greatest in New Mexico — 62 percent — and the overall amount of drugs captured was also far lower than in other states.

DEA officials caution that it is too early to say whether the rush of Central American migrants is responsible for the falling drug interdiction numbers, noting that the biggest narcotics loads in Texas are typically seized from vehicles at highway checkpoints farther north, not on the banks of the Rio Grande.

Customs and Border Protection spokesman Michael Friel said that his organization has “no indication that drug interdiction operations have been negatively impacted by our efforts to process the influx” of migrants. He said the Border Patrol’s Rio Grande sector has added more than 100 agents from other parts of the border that have helped its anti-drug work.

As the humanitarian crisis intensifies, U.S. officials are raising the pressure on their Mexican and Central American counterparts to halt the flow of migrants, many of whom are driven by violence, poverty and the perception that they will be allowed to stay if they reach U.S. soil. Secretary of State John F. Kerry raised the issue during a recent visit to Mexico. And Vice President Biden was headed to Guatemala on Friday to discuss a tightening of that country’s border with Mexico.

“Even if the VP says, ‘Don’t come,’ it’s going to be a tough sell,” said Eric Olson, associate director of the Latin America program at the Woodrow Wilson Center in Washington. There is a sense in these countries that “this is your big chance. If you want to get into the U.S., now is the time.”

But migrants can cross the Mexico-Guatemala border easily, often in plain sight of Mexican checkpoints. The boundary is in a sparsely populated region of thick jungle and mountains and is lightly patrolled, without the high-tech scanners, drones or imposing walls used on the U.S. border. The Mexican government does not seem interested in or capable of investing heavily to militarize its southern border, particularly with its security forces fighting drug cartels in several parts of the country and the challenge of Central American migration largely viewed as a problem for its wealthier northern neighbor.

“We’re not going to do the work of the U.S. on the southern border,” a senior Mexican official said on the condition of anonymity to speak candidly.

Mexico eased its immigration enforcement with a 2011 law that granted migrants new rights and no longer made it a crime to enter the country illegally. Mass kidnappings and other attacks against migrants by criminal gangs have emboldened activist groups to push Mexican officials to further expand protections for those transiting the country en route to the United States.

Amalia García Medina, a left-leaning congresswoman who is the head of the migration commission in Mexico’s lower house, said in an interview that she has proposed the creation of a “migrant visa” that would give U.S.-bound travelers safe passage and protection from deportation by the Mexican government for 30 days. A similar legislative proposal three years ago failed to pass.

“We shouldn’t be criminalizing those who are present in Mexico without documentation,” she said in an interview, noting that the migrants’ irregular status is what often forces them to risk their lives on freight trains or allows them to fall prey to abusive police officers. “We’re trying to protect the migrants from harm and guarantee their human rights.”

Mexican immigration officials are among the lowest-paid law enforcement officers in the country, and the federal government cannot afford to significantly tighten up the border, said Gustavo Mohar, a former senior Mexican immigration official.

“The government doesn’t have the resources,” he said. “And as you have proven in the U.S., you have invested billions of dollars in the Mexico-U.S. border, literally, and still people are able to avoid it.”

Gabriela Martinez in Mexico City contributed to this report.