The U.S. reaffirms its land border restrictions with Canada and Mexico, days after Canada relaxes theirs.

The United States will keep its land borders with Mexico and Canada closed for all nonessential travel for at least another month, the Department of Homeland Security said on Wednesday, extending their travel restrictions just days after Canadian officials announced they would soon reopen to U.S. travelers.

“To decrease the spread of Covid-19, including the Delta variant, the United States is extending restrictions on nonessential travel at our land and ferry crossings with Canada and Mexico through August 21,” a spokesman for the department said in an email. “D.H.S. is in constant contact with Canadian and Mexican counterparts to identify the conditions under which restrictions may be eased safely and sustainably.”

The United States closed land borders with its two neighbors in March 2020 as a way to limit the spread of the coronavirus. D.H.S. has renewed the restrictions every month since, in coordination with Canadian and Mexican authorities.

On Monday, Canadian officials said that American travelers could enter the country beginning on Aug. 9, assuming they have been fully inoculated with a vaccine approved by the Canadian government for at least 14 days.

Jen Psaki, the White House press secretary, told reporters on board Air Force One on Wednesday afternoon that “we rely on the guidance of our health and medical experts, not the actions of other countries” to determine border security policy.

Prime Minister Justin Trudeau of Canada said at a news conference on Tuesday that he thought that it was up to individual countries to decide their own border restrictions.

“Every country gets to set its own rules about how it will keep its citizens safe,” Mr. Trudeau said.

Nonessential travel restrictions to Canada and Mexico do not apply to air, freight rail or sea, and traveling by land is still allowed for many reasons, including business, medical purposes and education. All international air travelers into the United States have to present a negative coronavirus test taken within three days of departure or proof of recovery from the virus within 90 days.

Canada made the decision to reopen based on its vaccination progress — more than three quarters of the country had received at least one dose of the vaccine, according to Canadian governmental data, a far higher percentage than the United States, where a little more than 56 percent of the population has received at least one dose, according to the Centers for Disease Control and Prevention.

Residents on both sides of the border have pressed for reopening, and more than 2,800 people have joined a private Facebook group organized by Let Us Reunite, an advocacy group, to press for a more relaxed border policy.

One of the group’s members is Heather Kienle, an American citizen who lives in Montreal. Crossing the border has not been a problem for Ms. Kienle, but her Canadian husband, Etienne Bouchard, cannot.

So Ms. Kienle, who is six months pregnant, often drives alone or with her four-year-old daughter more than eight hours to West Babylon, N.Y., to care for her mother, who has endometrial cancer.

“It was just very stressful because I had to travel by myself, without my husband, and I had to take care of my daughter in the back seat,” Ms. Kienle said on Wednesday.

U.S. politicians from both parties have also objected to the restrictions.

Brian Higgins, a congressman who represents a district in Western New York that borders Canada, said in a statement on Wednesday that “today’s decision by the Biden administration harms economic recovery and hurts families all across America’s northern border; this is completely unnecessary.”

Mexican officials had recently expressed hope that the United States would agree to open up their shared border. A vaccination program in northern Mexico was ramped up as well. “We are working on this, so that the economic and social activity of the border can be regularized as soon as possible,” Mexico’s foreign minister, Marcelo Ebrard, told Forbes Mexico this month.

American citizens can travel into Mexico for any reason — to buy cheaper goods, access cheaper health care or because they live in Mexico and commute to work in the U.S. — but the border shutdown has meant many businesses have lost customers and been forced to close.

Also on Wednesday, President Biden nominated David L. Cohen, a lobbyist and senior adviser to the chief executive at Comcast Corporation, as ambassador to Canada. Mr. Biden has tried to demonstrate diversity in terms of the kinds of people he is appointing to high-profile postings abroad, but Mr. Cohen represents the old model of rewarding campaign donors with coveted posts abroad.

A Democratic fundraiser, Mr. Cohen hosted Mr. Biden’s first formal presidential event at his home in 2019 and helped him raise hundreds of thousands of dollars. Mr. Cohen was also the longtime chair of the board of trustees at the University of Pennsylvania, an institution that hired Mr. Biden for a lucrative university position after the Obama administration.

Annie Karni contributed reporting.