OTTAWA - Rejecting explanations from big oil companies, the opposition parties are demanding the federal government investigate why gasoline prices have been surging.

But there's little Ottawa can do to curb prices at the pumps, say the Conservatives. The NDP demanded a public inquiry after gasoline prices reached as high as $1.28 a litre in Vancouver on Tuesday, and only slightly lower in other parts of Canada. Pump prices have since dropped but remained just under $1.10 a litre in many regions.

"Yesterday, Imperial Oil reported a 31 per cent increase in quarterly profits, on the very same day that gas prices went through the roof," New Democrat Judy Wasylycia-Leis said in the Commons on Wednesday.

"So here we have big oil companies making big profits and consumers still paying big prices. It doesn't add up," she said.

"Why doesn't this government take on these big gas makers?"

Ottawa has limited ability to counter rising gasoline pump prices, said Natural Resources Minister Gary Lunn.

"There have been six federal studies of gas prices and each and every time the Competition Bureau has found there's been no price fixing," said Lunn.

"If the member has information and would like another investigation, they're welcome to bring that forward."

Lunn also cited Conservative government measures to mitigate the effects of higher gasoline prices.

"We brought in the $2-billion biofuels strategy," he told the Commons. "We're providing incentives for Canadians to purchase fuel-efficient vehicles ... and we have lowered the GST."

Rather than look at price fixing, the federal government should instead investigate why oil companies have been closing gasoline refineries, said Bloc MP Robert Vincent.

"The sharp increase in gas prices is not explained by international factors alone," he said. "It's also due to the intentional closure of some refineries.

"We know prices at the pump come under provincial jurisdiction, but can't the federal government check the profit margin of refining since that's under its jurisdiction?"

Oil companies and industry observers have blamed rising gas prices on everything from unrest in oil-rich Nigeria and tensions in the Middle East to high demand and low inventories. The cost of buying gasoline also moved higher in the United States, with an average gallon of gas at American pumps rising Tuesday to US$2.96, up nearly 80 cents US a litre.

The national average price of gas in Canada on Tuesday was just over $1.10 a litre, up nearly five cents from the average price in March, and 19 cents higher than the average price in January, said MJ Ervin and Associates Inc., a Calgary-based consulting firm.