Canada's main spy agency last year warned energy companies about an increasing risk of cyber espionage and attacks on pipelines, oil storage and shipment facilities and power transmission towers using homemade explosives, according to a classified document obtained by Reuters.

The Canadian Security Intelligence Service warning last May highlights an additional risk for the energy sector, where opposition to pipelines has ramped up in Canada, home to the world's third-largest oil reserves, and the United States.

In the document, which features speaking notes prepared for a CSIS briefing with energy and utilities sector stakeholders, an unidentified official specifies a threat from foreign state-owned firms looking for confidential information about investments or takeovers.