The Supreme Court of Canada will take time to weigh arguments about the constitutionality of an 18-year refugee agreement between Ottawa and Washington after hearing a challenge Thursday from claimants and human rights advocates.

The Canada-U.S. Safe Third Country Agreement (STCA) allows Canada to turn away asylum seekers seeking entry from the U.S. at official land border crossings.

However, human rights groups say the U.S. is not a "safe country" for asylum seekers and the pact allows Canada to skirt its international obligations for refugee claimants.

CTVNews.ca breaks down what the agreement entails.

WHAT DOES THE STCA DO?

The Canada-U.S. Safe Third Country Agreement was signed in 2002 and came into effect in 2004. Under the agreement, those seeking refugee status in either Canada or the U.S. must make their claim in the first country they enter.

That means most asylum seekers who attempt to cross into Canada at an official crossing are turned away and are told they need to make their asylum claim in the U.S., and vice versa. The only exemptions apply to unaccompanied minors and those with close family members living in Canada.

"If one of those narrow exemptions does not apply, you're not able to make a claim for refugee protection in Canada. And so what that means is that you're ordered to be removed or deported, and they contact U.S. authorities," Amnesty International's Julia Sande told CTV's Your Morning on Thursday.

But the agreement has one key loophole: it only applies to official land border crossings. That means that asylum seekers who manage to make a refugee claim within Canada while bypassing an official border crossing won't be sent back to the U.S.

This has prompted tens of thousands of asylum seekers to enter Canada at irregular crossings, such as Roxham Road, a rural road that goes through the border between Quebec and New York State.

HOW MANY ASYLUM SEEKERS HAVE CROSSED IRREGULARLY?

Since February 2017, Canada has seen enter the country. Of these, 28,332 (41 per cent) have had their refugee claims approved. In addition, 19,646 refugee claims have been rejected, 13,369 are still pending and the rest have either been withdrawn or abandoned, according to the Immigration and Refugee Board of Canada (IRB).

Irregular crossings into Canada surged after Donald Trump became president of the United States in 2017, as concerns grew over his anti-immigration rhetoric and executive orders limiting the number of refugees admitted.

According to data from the IRB, the number of irregular crossings peaked between July and September 2017. During this time period, 8,558 asylum seekers irregularly crossed into Canada, corresponding to an average of 2,853 per month.

The average number of irregular crossers per month dipped after that and hovered between 1,200 and 1,400 from late 2018 to early 2020. However, irregular crossings came to a near screeching halt after COVID-19 restrictions at the border were put in place in March 2020 and asylum seekers were sent back to the U.S. unless they met one of the exemptions.

In November 2021, as Canada continued lifting COVID-19 measures at the border, irregular crossers were once again allowed to enter the country and make a claim. Between April and June 2022, 4,512 irregular crossers entered Canada -- the most seen since 2019, according to the IRB.

WHAT DO OPPONENTS OF THE STCA SAY?

In 2017, Amnesty International, the Canadian Council for Refugees, and the Canadian Council of Churches of the Safe Third Country Agreement.

The organizations say the legislation underpinning the STCA violates Section 7 of the Charter of Rights and Freedoms, which guarantees life, liberty and security of the person, in addition to Section 15, which guarantees equal protection and benefit under the law.

Sande says asylum seekers who are turned back from Canada often face immigration detention in the U.S.

"When people are in detention, they're subjected to solitary confinement, staggering rates of sexual violence, really inhumane conditions, not given religiously appropriate food," she said. "The detention in itself is problematic and harmful. But in addition, when you're in detention, it's a lot more difficult to access counsel."

Sande says the increased difficulty accessing legal counsel means asylum seekers have a higher chance of being deported. On top of that, she said crossing the border at irregular crossings can come with serious risks.

Many of these crossers use Roxham Road, where the RCMP have set up a presence to handle the high volume of asylum seekers. But at other parts of the border, some asylum seekers have made long journeys on foot through empty farm fields in the winter, risking frostbite.

"We've heard of people losing fingers from frostbite and really putting themselves at risk. And so I would say it's neither compassionate nor orderly," Sande said.

Canada is also subject to the 1951 Refugee Convention, which stipulates that states cannot return refugees to dangerous countries. The human rights groups argue the pact lets Canada "contract out" its international obligations to refugee claimants without proper followup the U.S. is doing the job.

In July 2020, the Federal Court agreed, and ruled the Safe Third Country Agreement was unconstitutional. The federal government appealed the ruling and last December, the Supreme Court announced that it would hear the case.

WHAT HAVE FEDERAL PARTIES SAID?

The NDP and the Bloc Quebecois have long called on the federal government to suspend the Safe Third Country Agreement, and allow asylum seekers to cross into Canada at official crossings so they won't have to make potentially dangerous journeys through irregular crossings.

Meanwhile, the Conservatives say the STCA should be strengthened to allow Canada to send irregular crossers back to the U.S.

The three opposition parties calling for an inquiry looking at how public funds were used to build intake facilities at the border near Roxham Road.

In the House of Commons on Wednesday, Prime Minister Justin Trudeau said Canada "works with the U.S. government every day to improve the Safe Third Country Agreement." Immigration, Refugees and Citizenship Canada (IRCC) told CTV's Your Morning the agreement "has served Canada well" and is necessary to ensure that the border "remains well-managed."

"Canada believes that the STCA remains a comprehensive means for the compassionate, fair and orderly handling of asylum claims in our two countries," IRCC said in an email statement.

Quebec Premier Francois Legault has also called on the feds to and said his government does not have the capacity to deal with the influx of people. Public Safety Minister Marco Mendicino said on Thursday the government is working "very carefully with Quebec" to manage the flow of asylum seekers.

"We transfer significant federal funds to that province every year to help with ensuring that there is due process, that there... is a baseline of support for people who are filing claims," he told reporters before a cabinet meeting in Ottawa.

"We have to reach agreements, with partnerships with the United States, with Quebec, and that's exactly what the federal government will do," he added in French.

With files from The Canadian Press