The police confirmed the killings.
Gunmen on motorcycles on Monday morning shot dead three police officers.
The incident happened close to a village, Gidan Mutum Daya, in Kusada Local Government Area of Katsina State.
The village is a border community between Katsina and Kano.
A witness, Sa’adu Kabir, who was driving a commercial bus from Kano to Katsina, said it occurred at about 10:00 a.m.
Mr. Kabir said he had just driven his nine passengers past the police checkpoint where they were stopped by the police officers. After the search, he drove past the police a little and appealed to his passengers that he wanted to urinate.
Speaking in Hausa, Mr. Kabir told a PREMIUM TIMES correspondent that while he was urinating, they heard gunshots and as he looked back he saw the corpse of the policemen.
He said he and his passengers wanted to run into the nearby bush when the attackers told them not to.
“They told us they were not going to harm us; that they were just after the policemen,” he said.
The Katsina police spokesperson, Aminu Abubakar, confirmed the attack.
Mr. Abubakar, a Deputy Superintendent of Police, told journalists on phone that the attackers came in two motorcycles, each with two occupants, a rider and a passenger.
He said the victims were carrying out a basic stop and search operation when the attackers opened fire on them.
No group has claimed responsibility for the attack.
Katsina is in North-West Nigeria and is not one of the states largely associated with the Boko Haram insurgency.