Canada wants to change a bilateral agreement to allow it to turn back thousands of asylum seekers walking across the border but the United States is not cooperating, according to a Canadian official with knowledge of the discussions.

Nigerian asylum seekers rank among the highest in a new wave of asylum seekers fleeing the United States on account of President Donald Trump immigrant policy. Outside the United States, Canada has of late become the preferred destination of Nigerian asylum seekers and migrants. If Canada tightens the agreement with the US, the new deal would stifle efforts of these migrants at crossing from US to Canada in later months and years. Nigeria and Haiti head the list of highest migrants crossing from US to Canada.

Under the Safe Third Country Agreement, or STCA, asylum seekers who arrive at a formal Canada-U.S. border crossing going in either direction are turned back and told to apply for asylum in the first country they arrived in.

Canada wants the agreement rewritten to apply to the entire border.

More than 26,000 people have illegally crossed the Canada-U.S. border to file refugee claims in the past 15 months, walking over ditches and on empty roads along the world’s longest undefended border. Many have told Reuters they might have stayed in the United States were it not for President Donald Trump’s immigration rhetoric and policies.

Canadian officials first discussed changing the pact with U.S. Department of Homeland Security officials last September, shortly after more than 5,700 asylum seekers walked into Canada in August.

“We’d like to be able to get them to agree that we can, if somebody comes across, we just send them back,” the official told Reuters on Friday, adding Canada had raised the issue “at least a dozen” times since.

“I wouldn’t say they’ve been objecting or saying: ‘No, we won’t do it,’ but it’s been not responding rapidly.”

The Department of Homeland Security is reviewing Canada’s proposal and has not yet made a decision, a spokeswoman said.

The Canadian official compared Canada’s position to U.S. requests that Mexico prevent migrants traversing its territory from entering the United States

“We’ve got a problem, here. We’ve got to fix it,” the official added. “And we need the Americans’ cooperation.”

For now, another official said, Canada would keep doing what it is doing: Managing the influx of refugee claimants in a strained system, while seeking to dissuade would-be crossers through outreach efforts.

Even if the United States agreed to take back anyone trying to cross into Canada, keeping people out between all ports of entry would be a challenge and could result in asylum seekers taking potentially deadly risks to avoid detection, said University of Toronto law and human rights professor Audrey Macklin.

The STCA already faces a Canadian court challenge that argues the agreement is discriminatory and violates Canada’s Charter of Rights and Freedoms.

Canada has also urged U.S. officials to crack down on visas, saying many of the asylum seekers had valid U.S. visas and used the United States merely as a transit point.

Earlier this year, Canadian officials travelled to Nigeria, the source of a significant number of asylum seekers, to speak with Nigerian government officials and U.S. embassy staff.

The number of U.S. visas being issued to Nigerians has since dropped, said Mathieu Genest, a spokesman for Canadian Immigration and Refugee Minister Ahmed Hussen.