* Local official says unclear if killers from Boko Haram
* Islamist groups pose security threat in Nigeria's north
KADUNA, Nigeria Feb 26 (Reuters) - Gunmen have shot dead seven men who were guarding a market in northeast Nigeria's Yobe state, an official said on Tuesday, the latest attack in a region regularly targeted by Islamist insurgents.
At least 26 people have been killed in several attacks by gunmen in northern Nigeria since Friday, including the latest strike in the Kune region of Yobe.
Some of the attacks have been blamed on Islamist sect Boko Haram, a group that killed hundreds last year in an attempt to carve out an Islamic state in a country of 160 million mixed roughly equally between Christians and Muslims.
One faction of the group agreed a ceasefire last month in its home territory in northeast Borno state but violence there and elsewhere in the mostly Muslim north has continued.
"Yesterday night some people came with guns and killed seven people and injured one person, we can't tell if they are Boko Haram yet," said Maina Gana, Kune local government chairman.
"These people that were shot dead were vigilante men, they were shot while guarding the town's market."
Boko Haram and spin-off Islamist groups pose the greatest threat to stability in Africa's biggest oil producer and the risk to foreigners living in Nigeria is growing.
A group of seven expatriates were kidnapped from their construction compound in northern Bauchi state on Feb. 16. An Islamist group called Ansaru, which has taken foreigners before in Nigeria, claimed responsibility.
Three days later a French family of seven were abducted in northern Cameroon and taken over the border into Nigeria.
In a video showing the family posted online on Monday, a group saying it was Boko Haram said it would kill the hostages if its members in Nigeria and Cameroon weren't released. (Reporting by Isaac Abrak; Writing by Joe Brock; Editing by Michael Roddy)