Nigerian troops have rescued 13 hostages who were kidnapped by an extremist group in the northwestern state of Kaduna, the country's army said on Saturday.

The army said in a statement that "the troops successfully overwhelmed the terrorists, forcing them to abandon their captives."

Several kidnappers were killed, and others were captured, the military added. It didn't specify which armed group the kidnappers belonged to.

The rescued hostages were taken to a military facility for a medical assessment before being reunited with their families. Weapons, ammunition, solar panels and cash were also discovered during the rescue operation.

Kidnappings have become common in parts of northern Nigeria, where dozens of armed groups take advantage of a limited security presence to carry out attacks in villages and along major roads. Most victims are released only after the payment of ransoms that sometimes run into the thousands of dollars.

At least 1,400 students have been taken from Nigerian schools since the 2014 kidnapping of 276 schoolgirls by Boko Haram militants in the village of Chibok in Borno state shocked the world.

Boko Haram, Nigeria's homegrown jihadi rebels, launched its insurgency in 2009 to establish Islamic Shariah law in the country. At least 35,000 people have been killed and 2.1 million people displaced due to the extremist violence, according to United Nations agencies in Nigeria.