TORONTO (Reuters) - Canada has officials working with U.S. visa officers in Lagos, Nigeria, as Ottawa leans on its neighbor to stop issuing so many visas to Nigerians who then make refugee claims in Canada.

The Canadian government is trying to stem the flow of asylum seekers illegally walking across the U.S. border even as their ranks grow: About 2,500 asylum seekers crossed into Canada to file refugee claims in April, according to estimates from the federal immigration and refugee department -- the highest level since August and almost triple last April’s figure.

More than 26,000 people illegally crossed the Canada-U.S. border in the past 15 months to file refugee claims.

(Graphic: tmsnrt.rs/2HCp4aD)

The Canadian government says many of the more recent arrivals are Nigerians who arrived bearing valid U.S. visas after having spent very little time in the United States.

“It is apparent that they obtained those visas with the express intent to actually go to Canada. ... We’ve been sharing that information with the United States with the view of preventing the abuse of U.S. visas,” a Canadian immigration department spokeswoman told Reuters in an email.

Prime Minister Justin Trudeau and his ministers faced pointed questions this week after Reuters reported that Canada wants U.S. help turning back thousands of asylum seekers.

A Canadian official familiar with the matter told Reuters that Canada wants to amend a bilateral agreement to allow it to block border-crossing refugee claimants.

Canada has asked for this change “at least a dozen” times since September, the official said.

The U.S. Department of Homeland Security has said it is reviewing Canada’s proposal but has not made a decision.

Two Canadian officials have been sent to Lagos to work directly with their counterparts in the U.S. visa office, a spokeswoman for Immigration, Refugees and Citizenship Canada said in an email on Wednesday.

The officials are “meeting regularly to exchange information on migration movements” with the aim of lowering the number of people who go through the United States to Canada using a U.S. visa.

Since June, Canadian police have intercepted more than 7,600 Nigerian asylum seekers, 81 percent of whom had a valid U.S. non-immigrant visa, the spokeswoman added.

A spokeswoman for the U.S. State Department wrote that “consular officers in the field often coordinate with our close partners from other countries to discuss matters of shared concern.” She did not elaborate on the role the Canadian officials are playing.

Trudeau’s government is under pressure to appear in control of the country’s border and refugee system while obeying Canadian law and maintaining its image as compassionate and welcoming of newcomers.