* Second attack targeting police in under 24 hours
* Boko Haram the country's top security threat
* Nigerians fear Mali involvement could worsen insurgency
KANO, Jan 15 (Reuters) - Islamist gunmen shot dead four policemen on patrol in northern Nigeria's main city of Kano on Tuesday, the second attack in under 24 hours, ending a brief lull in violence.
A police officer on the scene after the attack, who declined to be named, said the gunmen ambushed the patrol vehicle before escaping on motorcycles - a classic tactic of Islamist sect Boko Haram.
After a surge in fighting around the turn of the year, there have been fewer assaults by the Islamist militants in the past two weeks, although they remain the biggest security threat to Africa's top oil producer.
"Four police officers attached to the Kano police command were shot dead today by suspected terrorists of the Boko Haram sect, near the Angwa-Ukwu police checkpoint," the police officer said. "They escaped on their motorbikes."
The attack came less than a day after two policeman were killed on Monday at a checkpoint elsewhere in Kano.
Boko Haram, based loosely on the Afghan Taliban, killed hundreds last year in a campaign to impose sharia, or Islamic law, on religiously mixed Nigeria.
The sect has forged links over the years with international jihadists in Mali and Niger, including al Qaeda's north African wing, and Nigerian officials fear the country's involvement in a proposed intervention force to fight Islamists in Mali could inflame its own Islamist insurgency.
Boko Haram's violence remains focused mostly on security forces in the northeast, although its attacks have spread across the north and to the capital Abuja.
A senior Boko Haram commander blamed for organising several suicide bombings, Mohammed Zingina, was captured on Sunday in Maiduguri in the northeast, where the sect's headquarters lie.
The government has promised rewards for information on some Boko Haram members that could lead to their arrest. Zingina was on that list.
Another, emerging group called Ansaru, known to have had ties with Boko Haram, claimed an attack on a police barracks in the capital Abuja in November, where it said hundreds of prisoners were released.
The group, labelled a terrorist organisation by Britain, has also said it was behind the kidnapping of a French national at the end of last year who is still missing. (Reporting by Chukwuemeka Madu; Writing by Tim Cocks)