WASHINGTON/CALGARY - Environmentalists shut down a Canadian oil sands mine on Tuesday in a series of protests on the eve of Prime Minister Stephen Harper's visit with President Barack Obama, aimed at pressing their case that the projects undermine the fight against climate change.

Green groups accused Harper's government of trying to hamper U.S. efforts to cut greenhouse gas emissions by seeking protections for Canada's oil sands industry, a major supplier of crude oil to the United States.

Harper meets with Obama at the White House on Wednesday, with climate and environmental issues expected to be on the agenda.

In northern Alberta, Royal Dutch Shell Plc suspended production at its Albian Sands Muskeg River mine after 25 activists from Greenpeace blockaded a massive dump truck and mining shovel to protest against oil sands development.

Shell, which owns 60 percent of the 155,000 barrel a day operation, said it temporarily shut down operation to ensure that the activists and its staff did not get hurt.

Shell's No. 1 concern is their safety and our preference is for a negotiated end to this demonstration, the company said in a statement. We have invited the group into our administrative building to sit down with management to discuss their concerns.

It said Greenpeace has not tried to contact Shell to discuss environmental initiatives it is employing at the site.

Numerous Canadian and U.S. environmental groups have intensified campaigns against oil sands development, which they say is damaging to air, land, water and local communities.

It's very clearly priority number one for our federal government to lobby the United States administration to give the tar sands a break, Rick Smith, executive director of Environmental Defense Canada, said on a conference call with reporters. Our hope as Canadians is that the U.S. administration resists our government's lobbying.

MASSIVE RESOURCE

At 173 billion barrels, Canada's oil sands represent the largest source of crude outside the Middle East. But unlike conventional production, the extra-heavy crude is mined in open pits and separated with hot water and chemicals, or heated with steam to allow it to flow to the surface.

Environmentalists warn of soaring carbon emissions with oil sands production expected to nearly double by 2015.

The industry counters that it is investing hundreds of millions of dollars in land reclamation and such new technologies as carbon capture and storage.

Smith said Ottawa has floated climate plans that would give oil sands producers special treatment and lobbied against U.S. provisions that would run counter to the industry's interests.

Obama has made addressing climate change a key mission. The U.S. House of Representatives passed legislation earlier this year to cap emissions by requiring companies to acquire permits for the greenhouse gases they emit into the atmosphere. But the bill has stalled in the Senate.

The U.S. Environmental Protection Agency and the Transportation Department also announced Tuesday proposals to strengthen vehicle fuel economy standards and reduce carbon emissions.

Despite these efforts, the White House has not taken a tough stance against the development of Canada's massive oil sands reserves.

Much to the chagrin of activists, the U.S. State Department recently approved Enbridge Inc's $3.3 billion Alberta Clipper pipeline project, which will mostly transport crude from the oil sands.

We should really be thinking about whether we want to be building an infrastructure that is going to lock us into an economic reliance on tar sands oils for next 20 to 50 years, said Susan Casey-Lefkowitz, Canada program director for the Natural Resources Defense Council.

Elsewhere on Tuesday, activists hung a 70-foot (21-meter) banner above Niagara Falls on the Canada-U.S. border showing arrows that point forward to a clean energy future and backward to tar sands oil.

(Editing by Rob Wilson)

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