Nigeria's president has said his government will do all it can to find those responsible for a string of bomb attacks that killed at least 32 people near the central city of Jos.
Goodluck Jonathan promised that the bombers would "face the law". No group has said it carried out the attacks.
The Christmas Eve blasts happened in an area where up to 1,000 people died in sectarian clashes this year.
The region straddles Nigeria's mainly Muslim north and the Christian south.
About 74 people were wounded in the bomb blasts. Some are in a critical condition.
In a separate development, at least six people died in attacks on churches by suspected Islamists in the north-eastern city of Maiduguri.
Speaking in the capital Abuja, President Jonathan said: "I assure all Nigerians that we shall unearth those behind the Jos bomb explosion and apprehend them to face the law."
A statement from his office added: "The president expressed sadness at explosions which killed many innocent Nigerians, Christians and Muslims alike."
Friday night's bomb explosions occurred during Christmas Eve celebrations in villages near Jos, police say.
Gregory Yenlong, a spokesman for Nigeria's Plateau state, told Bloomberg news agency that there had been threats "to disrupt Christmas celebrations in Jos".
Past clashes have been between rivals gangs of Muslims and Christians, but observers say the underlying causes are economic and political rather than religious.
Muslims are generally from the Hausa- or Fulani-speaking communities. They are often nomadic people who earn a living from rearing animals or petty trade.
The mainly Christian Berom, Anaguta and Afisare groups have traditionally been farmers.
Some Christian farmers feel they are under threat, as Hausa-speaking Muslims come down from the north looking for pasture for their animals.
In Maiduguri, suspected Islamist sect members attacked at least two churches late on Friday.
In one incident, petrol bombs killed five people including a Baptist pastor, Reuters news agency reported.
A security guard at a nearby church died in a similar assault, Reuters added.