Ontario's provincial utility says power has been restored to more than half a million customers four days after an ice storm first moved into the region, but that some hard-hit areas may remain without it until the end of the week.

Hundreds of thousands of Hydro One customers are without power after a massive winter storm swept through Ontario, causing widespread damage.

Over 300,000 Canadians faced power outages in parts of Ontario on Sunday as an ice storm pummeled the region over the weekend, according to electricity provider Hydro One.