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Two American and two Canadian citizens have been kidnapped in the northern state of Kaduna, Nigeria, police say.

Two police escorts were also killed in a shootout with the kidnappers, who emerged from hiding to attack the convoy.

The group, travelling in two vehicles, was reportedly attacked on their way from Kaduna to the capital Abuja, about two hours away.

Parts of the Abuja-Kaduna road have been target locations for kidnappers.

John Babcock, a spokesman for Global Affairs Canada, told the BBC on Wednesday they were aware of the reports of the kidnapping.

"Consular officials in Nigeria are in contact with local authorities to gather additional information," he said.

A US State Department official said that they were "aware of reports of a US citizen kidnapped in Nigeria", but said the department currently had no further comment.

Two German archaeologists were abducted in the region in February last year. They were later freed.

There has been an increased spate of kidnappings in Nigeria, where both Nigerians and foreigners have been targeted.

Last October, four Britons were abducted in the Niger Delta region. One of them was killed while three others were released after negotiations.