* Third attack targeting police in two days
* Nigerians fear Mali involvement could worsen insurgency
Nigerian forces killed two Islamist gunmen in a shootout and arrested five others after their checkpoint in the north's main city of Kano was attacked on Thursday, they said.
Kano has seen a spate of shootings this week attributed to the Islamist militant group Boko Haram, based loosely on the Afghan Taliban.
The group killed hundreds last year in a campaign to impose sharia, or Islamic law, on religiously mixed Nigeria. A crackdown last year weakened it but also pushed it into new areas, and it remains the biggest security threat to Africa's top oil producer.
The Islamists have forged links over the years with international jihadists in Mali and Niger, including al Qaeda's north African wing, which French forces are now trying to dislodge from northern Mali.
Nigeria sent the first company of a planned deployment of around 900 troops on Thursday to join that campaign as part of a West African intervention force, and officials fear Nigeria's involvement could inflame its own Islamist insurgency.
Islamist gunmen shot dead four policemen in an ambush on their patrol in Kano on Tuesday. The previous day, two policemen were killed at a checkpoint in the city.
Boko Haram's attacks are focused mostly on security forces in the northeast, although they have spread across the north and to the capital Abuja.
Mohammed Zingina, a senior Boko Haram commander accused of organising several suicide bombings, was captured on Sunday in the northeastern city of Maiduguri, where the sect has its headquarters.