The Nigerian army says it has arrested two high-ranking members of the armed Islamist group Boko Haram.

A third man was killed during the operation on the outskirts of the city of Kano.

There were unconfirmed reports that the man who died was a Boko Haram spokesmen who uses the alias Abul Qaqa.

The AP news agency reported that two members of Boko Haram were killed in the incident.

According to a soldier who spoke on condition of anonymity, the three men were coming into Kano early on Monday along with a woman they were transporting to receive medical treatment.

One of those arrested is an alleged "field commander" for Kogi and Kaduna states as well as for the capital Abuja, the soldier said.

The woman who was with them allegedly told security forces that the man killed was Abul Qaqa.

Statements are often issued on behalf of Boko Haram in the name of Abul Qaqa, and someone identifying himself by that name has regularly held phone conferences with journalists.

Earlier this year, security sources said a suspect believed to be Abul Qaqa had been arrested.

At the time, a purported Boko Haram member confirmed one of the group's high-ranking members was arrested, but refuted reports that the detained person was its spokesman.

Boko Haram has been blamed for more than 1,400 deaths in Nigeria's northern and central regions.

The group's attacks have grown increasingly deadly and sophisticated, including suicide bombings at UN headquarters in Abuja and an office for one of the country's most prominent newspapers.