OTTAWA -- Canada’s shared border with the United States has officially closed to non-essential travel as part of a mutual effort to prevent the spread of the novel coronavirus.

Both countries had been negotiating a mutual ban on recreational travel without restricting the flow of two-way trade and commerce. The closure took effect at 12:01 E.T. on Saturday.

Essential cross-border workers like health-care professionals, air crews and supply chains will be permitted to cross. However, anyone attempting to cross the border for tourism or recreational reasons will be turned away.

The measure will be in place for 30 days.

In his remarks on Friday, Prime Minister Justin Trudeau said irregular migrants will also be turned back to the U.S. when they reach the Canadian border.

The new policy is part of an agreement with the U.S. that addresses concerns about the difficulty of screening refugee claimants for COVID-19 when they arrive.

"We will now be returning irregular migrants who attempt to cross anywhere at the Canada-U.S. border," Trudeau said, speaking to reporters outside of Rideau cottage in Ottawa, where he is self-isolating.

"We’re making sure that this behaviour and the process is aligned with Canada’s values."

Those who have already crossed into Canadian territory in recent days will be allowed to stay and continue through the asylum process, but are in isolation for the mandated 14-day period, said Trudeau.

He added that the announcement on Friday is a "temporary" measure that’s required in this unprecedented time.

Public Safety Minister Bill Blair doubled down on the government’s stated goal of protecting the health and safety of Canadians, and said there will be only "necessary and limited" exceptions.

An exception could, for instance, include an unaccompanied minor with American nationality, said Blair.

"We have over the past few weeks announced progressive and significant measures to restrict travel across our borders. These decisions, I want to assure everyone, have not been made lightly," said Blair in a subsequent press conference in Ottawa.

Canada has seen an uptick in irregular border crossers at unofficial entry points since early 2017, when U.S. President Donald Trump announced he would crack down on illegal immigrants. The Safe Third Country Agreement states that migrants arriving at official border checkpoints can be sent back and told to apply for asylum in the country they first arrived in, which is why many choose to enter at unofficial crossings.

Blair said Canada has seen on average about 45 to 50 people crossing over each day, on Thursday the number dipped to 17.

Friday’s measure temporarily seizes this activity.

"I think people are recognizing that this is not a time to be going to the border and crossing over and engaging in that behaviour because it presents a risk to them and to others."

Blair noted that Canadians in “extraordinary circumstances” will be given leeway at the border.

"Officials on both side of the border are working very closely to make sure that there is a consistency in the way in which these measures will be implemented," said Blair. "There is an expectation that border officers will exercise the appropriate discretion in determining those in exceptional and extraordinary circumstances if the travel is in fact essential."

The government has noted trade will continue, to prevent an even greater economic dip and to ensure essential goods flow from country-to-country.

Minister Blair said Friday he is encouraging truckers still crossing the border regularly to practice recommended hygiene measures and self-monitor for COVID-19 symptoms.

Canada has already shut its borders to most non-citizens looking to enter the country. International flights have been rerouted to four airports where more intensive screening is being conducted on arriving travellers, and those with symptoms are being denied boarding on flights to Canada.

If a passenger demonstrates symptoms of the virus upon arriving in Canada they are being taken under the care of public health officials, and everyone arriving in Canada from abroad is being asked to acknowledge they are to self-isolate for 14 days.

With files from Rachel Aiello, Nicole Bogart and The Canadian Press