Gunmen abducted four Catholic nuns on a highway in Nigeria's oil-producing Imo state in the southeast, a local convent said on Monday, in the latest sign of widespread insecurity making road travel unsafe.

Armed gangs have been kidnapping people, including priests, for ransom from villages and on highways mainly in the northwest and the practice has spread to other parts of the country, increasing insecurity in Africa's most populous nation.

Zita Ihedoro, secretary general of Sisters of Jesus, the Saviour Generalate, said the four were abducted while travelling from Rivers state to Imo for a thanksgiving mass on Sunday.

"We implore for intense prayer for their quick and safe release," Ihedoro said in a statement.

In the northwest, Nigeria's military has started an air offensive to eliminate the armed groups responsible for kidnapping citizens from villages and towns in the region.