Two explosions Monday rocked two different Nigerian cities repeatedly hit by attacks blamed on Islamist extremist group Boko Haram, wounding at least four people, police and residents said.

The explosions occurred in Kano, the largest city in northern Nigeria, and Maiduguri, the northeastern city at the heart of Boko Haram's insurgency.

A blast outside a mosque near Kano's main textile market went off around 8:30 am in an area where traffic police were stationed, residents said. The area was later cordoned off by troops and policemen, an AFP reporter said.

Police said the blast was caused by a "time-bomb" and left three traffic police and a passerby wounded.

"There was an explosion around the Kantin Kwari market targeting some traffic personnel," police spokesman Magaji Majia said.

"Four people were injured. They included a federal road safety personnel, a Hisbah personnel and a state traffic warden as well as a passerby."

The Hisbah are the local sharia police.

Residents of Maiduguri reported a huge explosion but did not yet know details.

"There was a very heavy explosion this morning that shook the entire city," a resident said.

Violence linked to Boko Haram's insurgency has left some 3,000 people dead since 2009, including killings by the security forces.