OUAGADOUGOU (Reuters) - Gunmen killed about 15 people in an attack on a mosque in northern Burkina Faso during prayers on Friday evening, a security source and a local official told Reuters.

The gunmen entered the mosque in the village of Salmossi in the Oudalan region bordering Mali and opened fire, the two sources said on Saturday. Others were seriously wounded, the security source said.

The identities of the gunmen were not yet clear.

An Islamist insurgency by groups with links to Islamic State and al Qaeda has crossed into Burkina Faso this year from neighboring Mali, igniting ethnic and religious tensions, especially in northern regions.

The attacks have shattered a relative calm in Burkina which, until this year, had largely been spared the kind of violence which has made large areas of the Sahel ungovernable, especially Mali.

The number of people forced to flee their homes in the worst-hit areas has increased more than six-fold since January to around 500,000, the United Nations and aid groups said on Friday.

Last week, 20 people were killed in an attack by suspected jihadists on a gold-mining site in the north.