Two American and two Canadians who were kidnapped in an ambush by gunmen in northern Nigeria this week were freed on Saturday, police said.

Nigerian security forces had launched a manhunt after the four were seized on Tuesday evening in the state of Kaduna by kidnappers who shot dead two of their police escorts.

"They have been rescued thanks to the efforts of the police," Kaduna state police spokesman Muktar Aliyu said, adding that they comprised three men and one woman.

"All of them are in a good condition of health" and are now in the care of their embassies, he said.

Aliyu said they were rescued at about 5 am (0400 GMT) in the same area where they were kidnapped, but declined to give further information, saying it was "classified information".

"I cannot confirm if there have been negotiations or a ransom paid," Aliyu added.

One person suspected of links to the kidnapping -- the latest abduction targeting foreigners in Nigeria -- has been arrested, he said.

The four North Americans -- whose identities have not been disclosed -- were on private business in Kaduna when they were snatched on the road from the town of Kafanchan to the capital Abuja.

"We are aware of reports of two US citizens kidnapped and released in Nigeria. The safety and security of US citizens overseas are among our top priorities. Due to privacy considerations we have no further comment," a US State Department official told AFP.

Kidnapping has long been a problem in Nigeria's southern states, where high-profile individuals, including the families of prominent politicians, are regularly seized.

- Violent crime common -

But as the economy stalled in recent years, the crime began creeping north.

A State Department travel advisory urges US citizens to "reconsider" travelling to Nigeria, warning that "violent crime such as armed robbery, assault, carjacking, kidnapping and rape is common throughout the country".

A crackdown on cattle rustling has been blamed for rising numbers of abductions in the north, with criminals turning to kidnapping.

The Kaduna-Abuja road is notoriously unsafe. It is a journey of about two-and-a-half hours by car through villages and past tracts of fields and forests.

Security on the route came under intense scrutiny last year when the federal government announced the closure of the capital's only airport for essential runway repairs.

Many foreign missions and companies advised staff to limit their travel during the closure period, as all domestic and some international flights were switched to Kaduna.

In July 2016, Sierra Leone's defence attache to Nigeria was kidnapped by men in military fatigues armed with AK-47 rifles at a fake checkpoint on the Abuja-Kaduna road.

And in another abduction against foreigners, in October, four Britons including a man and his wife working for a Christian charity were kidnapped in the southeastern Delta State.

The British government announced the following month that one of the hostages was killed, but the three others were freed.