* Attack is deadliest in Nigeria's northeast in 2017
* Bears hallmarks of Boko Haram faction that targets civilians
* Total northeast death toll in 2017 at least 278 -Reuters calculations (Adds details from witnesses)
By Percy Dabang and Ardo Hazzad
YOLA, Nigeria, Nov 21 (Reuters) - A suicide bomber killed at least 50 people at a mosque in northeastern Nigeria on Tuesday, the biggest mass killing this year in a region facing an insurgency by Islamist militant group Boko Haram.
"Some of the dead were in pieces beyond recognition," said Bayi Muhammad, a worshipper at the mosque in Mubi who said he only escaped the blast because he was late for early morning prayers.
The government and military have said numerous times since 2015 that Boko Haram's eight-year insurgency is almost defeated but the group continues to attack civilian and military targets.
Tuesday's bombing brings the number killed in 2017 to at least 278, according to calculations by Reuters. There was no claim of responsibility for the attack.
Abubakar Othman, a police spokesman in Adamawa state, said the death toll is at least 50 but "there could be more as those seriously injured could add to the figure."
Mubi is in Adamawa state where Boko Haram held territory in 2014. Troops pushed them out in early 2015. The group mounts suicide attacks in public places such as mosques and markets.
Tuesday's attack was the biggest in Nigeria's northeast since two schoolgirl suicide bombers killed 56 people and wounded dozens more at a market in Adamawa last December.
It is also the first attack on Mubi since armed forces recaptured the town from Boko Haram in 2014.
The blast bears the hallmarks of a faction led by Abubakar Shekau, which forces women and girls to carry out suicide bombings. The attacks often leave only the bomber dead.
Boko Haram has waged an insurgency in northeastern Nigeria since 2009 in its attempt to create an Islamic state in the region. It has killed more than 20,000 and forced around 2 million people to flee their homes.
The group split in 2016 and the faction under Shekau is based in the Sambisa forest on the border with Cameroon and Chad and mainly targets civilians with suicide bombers.
The other faction is based in the Lake Chad region and led by Abu Musab al-Barnawi. It mainly attacks military forces after quietly building up its strength over the past year.
Most attacks focus on Borno state, the birthplace of the insurgency. The group held land around the size of Belgium in Borno, Adamawa and Yobe states until early 2015 but was forced out by Nigeria's army and troops from neighbouring countries.
(Reporting by Percy Dabang in Yola and Ardo Hazzad in Bauchi; Writing by Alexis Akwagyiram and Paul Carsten; Editing by Matthew Mpoke Bigg)
Our Standards: The Thomson Reuters Trust Principles.