Both of Montana's senators criticized the Biden administration's decision on Friday to continue blocking nonessential travel from Canada into the United States for an 18th straight month.

The U.S. Department of Homeland Security announced Friday the border will remain closed to some travelers from Canada at least through Sept. 21 to minimize the spread of the coronavirus, including the highly contagious delta variant that has contributed to the latest surge in infections and related hospitalizations.

DHS said it would continue "to ensure the flow of essential trade and travel" while the northern border, as well as the southern border with Mexico, are otherwise closed to outsiders. The restrictions don't apply to air, freight rail or sea travel between the United States and Canada, but they do apply to travel via passenger rail, passenger ferries and pleasure boats.

"In coordination with public health and medical experts, DHS continues working closely with its partners across the United States and internationally to determine how to safely and sustainably resume normal travel," the agency said on Twitter.

Canada began welcoming fully vaccinated U.S. citizens and permanent residents on Aug. 9.

U.S. Sen. Jon Tester, a Democrat, called on the Biden administration to institute a similar policy to welcome vaccinated Canadians.

"Montanans depend on cross-border trade and travel to meet their bottom lines and keep their businesses afloat, as well as to stay in contact with family and friends in both countries who haven't seen each other in person for more than a year," Tester said in a statement Friday. "I urge the Biden administration to reverse this disappointing decision and reopen the northern border as quickly as possible to vaccinated Canadian travelers so that both of our economies can continue to fully rebound."

U.S. Sen. Steve Daines, a Republican, repeated his assertion that the southern border has been "wide open" to undocumented immigrants traveling from Mexico and accused Biden of "hypocrisy" for keeping travel restrictions in place.

"President Biden has once again let down Montana families and businesses. His refusal to open the northern border is hurting our economy, jobs and border communities," Daines said in a statement. "Since President Biden won't do what's right for Montana, it's time Congress pass my bill to reopen the northern border immediately."

A Daines spokeswoman said in an email that his legislation "would reopen the border regardless of vaccination status, but at the very least, the administration should reciprocate with the Canadian government and allow fully vaccinated Canadians to enter the country."

As of Aug. 9, U.S. citizens and permanent residents who currently reside in the United States may enter Canada if they have received a full series of an approved vaccine — Pfizer, Moderna, AstraZeneca or Johnson & Johnson — at least 14 days before arrival. Unvaccinated children younger than 12, and unvaccinated children who are dependent due to a mental or physical condition, may accompany a fully vaccinated parent, stepparent, guardian or tutor across the border. Travelers from the United States also must meet coronavirus testing requirements and be asymptomatic upon arrival.

Canadian air travelers to the United States must submit a negative coronavirus test no more than three days before departure, or proof from a health-care provider that they have recovered from COVID-19 in the past 90 days. The same applies to entry by sea or rail, with the exception of commuter trains and ferries.

U.S. citizens and permanent residents returning to the United States is considered essential travel and is not restricted.