The Nigerian Armed Forces have joined the hunt for two Americans and two Canadians who were kidnapped in an ambush, a military source told AFP thursday.

Kidnappers seized the four on Tuesday evening in Kaduna State, and killing two of their police escorts in the latest abduction targeting foreigners.

“We are on it and making some progress,” said the military source in Kaduna, on the condition of anonymity.

“We have a special team now that arrived from Abuja, it has now joined with the team that is here.”

The North Americans were on a private business visit in the state and were kidnapped near Jere, the source added. Police said unidentified armed men seized the four on the road from the town of Kafanchan to the state capital of Abuja at 7:00 pm on Tuesday.

The US and Canadian foreign ministries both said they were aware of the kidnapping and were working with Nigerian authorities on recovering the hostages.

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”.

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

But as the economy stalled in recent years, the crime began creeping north. 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 an approximately two-and-a-half hour journey by car through villages and past tracks of fields and forests.

Safety 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 attaché to Nigeria was kidnapped by men in military fatigues armed with AK-47 rifles at a fake checkpoint on the Abuja-Kaduna road.