Soldiers and policemen are pictured at the scene where two female suicide bombers blew themselves up at a mobile phone market in the northern Nigerian city of Kano November 18, 2015. [Photo/Agencies]

Since losing most of the territory they took over earlier this year to the Nigerian army, the militant group has focused attacks on markets, bus stations and places of worship, as well as hit-and-run attacks on villages.

Suspected Boko Haram militants have carried out attacks in neighbouring Chad, Niger and Cameroon in recent weeks but until Tuesday had not struck northeastern Nigeria since late October.

In Washington, the State Department condemned "horrific" bombings in the northern towns of Yola and Kano in recent days, and said the United States was committed to working with Nigeria and its neighbours to defeat Boko Haram.

"We denounce the callous terrorist acts," spokesman John Kirby said, adding: "Those responsible for these crimes must be held accountable."