Mexico, for example, allows US travelers by air, but there are still limits on who can cross the land border.

"We've had situations where folks aren't able to get clearance to cross on foot or by car, but they can easily go to an airport and fly," said San Diego Mayor Todd Gloria, who participated in last Thursday's call with US immigration officials and characterized it as professional. "It's these sorts of things that aren't consistent but add frustration."

The restrictions, which are mutually agreed upon with Mexico and Canada, are renewed monthly. The latest restrictions, announced in June, will remain in effect until July 21.

The Biden administration has not provided a time line for when those restrictions will lift, raising concern among border communities that rely in part on cross-border travel to fuel their economies.

"Our economy will not be reopened until our border is," Gloria said.

Internally, US Customs and Border Protection officials are bracing for the eventual lifting of border restrictions, according to two agency officials. Some are concerned about staffing and whether there are enough agents and officers to process an increased number of individuals, one of the officials said.

"We don't have the manpower, and that is the reality," the official told CNN.

Customs and Border Protection's Office of Field Operations, which is responsible for border security at US ports of entry, has pulled back officers to the ports in anticipation of restrictions being lifted, another agency official said.

Democratic Rep. Henry Cuellar of Texas has repeatedly asked officials when restrictions are expected to lift, both privately and in congressional hearings.

"There's not so much transparency on the US side," Cuellar told CNN. "They think it's imminent but there are no dates."

A Department of Homeland Security spokesperson told CNN, "To reduce the spread of COVID-19, the United States extended restrictions on non-essential travel at our land and ferry crossings with Canada and Mexico through July 21, while ensuring access for essential trade & travel. DHS also notes positive developments in recent weeks and is participating with other U.S. agencies in the White House's expert working groups with Canada and Mexico to identify the conditions under which restrictions may be eased safely and sustainably."

Democratic Rep. Brian Higgins of New York echoed frustrations over the lack of transparency on the easing of restrictions. "It defies logic. It defies science. It defies fact," Higgins, who also serves as co-chair of the Northern Border Caucus, told CNN, referring to the ongoing limitations on cross-border travel. He also stressed that border communities in the US and Canada are inextricably linked.

Republican Rep. Elise Stefanik of New York implored the Biden administration to lift nonessential travel restrictions in a June letter. "Our communities cannot afford any further delay or acquiescence -- unilateral action to begin reopening the border appears necessary," she wrote in the letter to Homeland Security Secretary Alejandro Mayorkas.

Last Friday, Canadian Prime Minister Justin Trudeau said his country continues "to work very closely with the United States to make sure that we're aligned in our approach" to keeping both countries' citizens safe. He added that the date of reopening will depend on how things go after Canada eased its mandatory two-week quarantine requirement Monday for fully vaccinated nationals and residents who travel abroad.

The nonessential travel restrictions are one part of a web of limits related to the coronavirus pandemic that remain in effect and have contributed to confusion and frustration.

The Biden administration has also faced fierce criticism for relying on the public health authority Title 42, which was put in place under the Trump administration to expel migrants encountered at the US-Mexico border. Since last October,

US Border Patrol has expelled 648,185 migrants under the authority, according to agency data.

Immigrant advocates claim the policy has put migrants in harm's way, leaving many, including those seeking asylum, in dangerous conditions in Mexico. In some cases, families have opted to separate from their children, since unaccompanied migrant children are not subject to the policy.

Public health experts sent a letter to Health and Human Services Secretary Xavier Becerra and Centers for Disease Control and Prevention Director Dr. Rochelle Walensky last week arguing that the order is "based on political, rather than public health, considerations."

Officials are considering ending the public health order, though it remains a point of contention among advocates and the administration.

Immigrant advocates raised the public health order and the urgency behind ending it during a roundtable with Vice President Kamala Harris last month in El Paso, Texas, according to Taylor Levy, an immigration attorney based in California who works with migrants along the southern border.

"Title 42 has truly been a basically Covid stimulus plan for the cartels, because they're able to make so much money," Levy said. Cartels have taken advantage of those expelled to Mexico under the authority, she added.

A Human Rights First report found that as of June 17, there had been more than 3,000 kidnappings and other attacks, including rape and human trafficking, against asylum seekers and migrants turned back at the US-Mexico border since President Joe Biden took office.

"It's not just about ending Title 42. It's also ensuring people are welcomed, received and allowed to proceed with their asylum cases in a way that supports them and ensures they have an opportunity to seek protection," Kennji Kizuka, associate director at Human Rights First, told CNN.