Eyewitnesses told the BBC the attack targeted a group of about 20 politicians at an outdoor cafe.
They had travelled to Dusa Mareb in Galgadud region - which is controlled by a pro-government militia - to promote reconciliation.
The Islamist militant group al-Shabab has said it was behind the attack.
Al-Shabab is under pressure on a number of military fronts in the south of Somalia - but still mounts frequent attacks and controls much of the country.
Al-Sunna Wal Jama'a, the militia which controls much of Galgadud, said the suicide attacker was among the dead.
Several other MPs and prominent politicians - including former Security Minister Ahmed Abdi Salam - were also injured in the attack, officials told the BBC.
Two civilians and two security guards were also killed.
Al-Shabab, which merged with al-Qaeda in February, phoned the BBC Somali Service to say it was behind attack on the tea shop where the politicians were resting after holding meetings with residents on Tuesday morning.
MP Dahir Amin Jesow, who was part of the delegation, told the BBC the attacker was a young man who approached the group casually chewing the mild narcotic leaf khat.
Once he was among the group, he detonated an explosive device shouting "Allahu Akbar (God is great)".