ST JOHNSBURY, Vermont (AP) — The immigration deal expected to be announced Friday by US President Joe Biden and Canadian Prime Minister Justin Trudeau would end a process that has enabled tens of thousands of immigrants from across the world to move between the two countries along a back road between New York state and Quebec.

Since early 2017, these migrants have entered Canada via Roxham Road outside Champlain, New York, where a reception centre staffed by agents of the Royal Canadian Mounted Police has been set up to process them, about five miles (8 kilometres) from the official border crossing where they'd be returned to the United States.

The Mounties warn that they'll be arrested if they take one more step. Then they do — and without being handcuffed, they are processed and usually released to live in Canada while their asylum cases are pending, which can take years.

Trudeau's government is expected to announce as part of the agreement that 15,000 migrants from the Western Hemisphere will be given slots to apply to enter Canada. The policy applies to people without US or Canadian citizenship who are caught within 14 days of crossing the border, and will enable both countries to turn away asylum seekers at their borders.

The change will take effect a minute after midnight Saturday, a quick implementation aimed at avoiding a surge of refugee claimants trying to cross the border, according to Canadian officials who spoke on condition of anonymity to discuss the deal before its formal announcement.

The deal comes as the US Border Patrol also responds to a steep increase in illegal southbound crossings along the wide-open Canadian border. Nearly all happen between the biggest population centres in both countries, from Quebec into upstate New York and Vermont.

While the numbers are still tiny compared to the US-Mexico border, it's happening so frequently now that the Border Patrol increased its staffing in the region and has begun releasing some migrants into Vermont with a future date to appear before immigration authorities.

Canadian officials have struggled to cope with this since early 2017. Many northbound migrants said they were fleeing because they feared President Donald Trump's immigration policies were hostile to their presence in the United States. The process continued since the Biden administration took office.

These migrants have taken advantage of a quirk in a 2002 agreement between the US and Canada that says asylum seekers must apply in the first country they arrive in. Migrants who go to an official Canadian crossing are returned to the US and told to apply there. But those who reach Canadian soil somewhere other than a port of entry are allowed to stay and request protection.

Meanwhile, southbound migrants are straining US border officials.

US Border Patrol agents stopped migrants entering illegally from Canada 628 times in February, more than five times the same period a year earlier. Those numbers pale compared to migrants entering from Mexico – where they were stopped more than 220,000 times in December alone — but it is still a massive change in percentage terms.

In the Border Patrol’s Swanton Sector, which stretches across New Hampshire, Vermont and a portion of upstate New York, agents stopped migrants 418 times in February, up more than 10 times from a year earlier. About half entering from Canada have been Mexicans, who can fly visa-free to Canada from Mexico.