Budget cuts have hampered the U.S. Border Patrol's work in its busiest sector on the Southwest border, agents said Friday, with the agency introducing fuel conservation measures in the Rio Grande Valley that have agents patrolling on foot and doubling up in vehicles.

The Border Patrol instituted the changes after the across-the-board government spending cuts known as sequestration. The constraints come as Congress moves deeper into the debate over comprehensive immigration reform and Republican legislators push for stronger border security components as a precursor to any path to citizenship for immigrants who have entered the country illegally.

The Rio Grande Valley sector — a stretch of border from Brownsville to Laredo on the southernmost tip of Texas — has become the agency's hottest area along the border. The Border Patrol's arrests of people trying to cross the border illegally jumped 65 percent in that area last year. At the end of May, sector Chief Patrol Agent Rosendo Hinojosa said agents had already made more than 90,000 apprehensions in the first eight months of the fiscal year, a 50 percent increase over the same period last year.

In a prepared statement Friday, the Border Patrol said, "Sequestration continues to have serious impacts on (Customs and Border Protection's) operations including nearly $600 million in cuts." A spokesman declined to address the fuel restraints specifically.

But a February letter from U.S. Customs and Border Protection to the union representing Border Patrol agents outlined cost-cutting measures the agency expected to implement. Among them were stopping Border Patrol supervisors from taking their vehicles home and assigning two agents to every vehicle instead of one. "CBP will also make reductions in fleet fuel and maintenance costs, as well as longer life-cycles for vehicles," the letter said.

An undated internal memo published Thursday by KRGV-TV in Weslaco indicates some of those steps and more have been taken. The Border Patrol would not comment on the authenticity of the memo, but agents in the sector confirmed Friday that the agency had cracked down significantly in the past month on fuel use.

Some agents are being shuttled into the field and then patrol on foot, while others are paired up in vehicles, an official with the agents' union, the National Border Patrol Council, said.

"If you're putting the same amount of people out there in the field but in less vehicles you're creating a smaller footprint, therefore leaving areas open," said Christopher Cabrera, vice president of the local National Border Patrol Council.

Officer safety becomes an issue because agents on foot can't respond in the same way as those in vehicles and backup can take longer to arrive, he said.

The impact is felt not only in the Border Patrol's own operations, but also in the border communities.

"Some of these communities out there, sometimes we're the only law enforcement presence they have out there and for us not to be there hurts," said Cabrera, an agent stationed in McAllen.

"Local police departments who do great job of helping us out, now they're going to be taxed even further because they're having to take up for our slack," he said. "More pursuits are coming through the area because these vehicles that are loading up with people and narcotics are moving forward from the river area because we don't have the presence down there and they're taking up the slack."

Homeland Security Janet Napolitano has said the department has taken notice of the spike in apprehensions along the border in the Rio Grande Valley and is making sure additional resources are available to Border Patrol agents in the area.

"We know the traffic is higher now. Actions are being taken to turn that traffic back," Napolitano told a Senate panel in April.

More than 2,500 Border Patrol agents are in the sector and the agency has said it's sending most of its new academy graduates to the Rio Grande Valley.