By Laura Sanicola and Nia Williams

NEW YORK, Feb 1 (Reuters) - Environmentalists and labor unions that threw their support behind U.S. President Joseph Biden now find themselves on the opposite sides of a battle over the construction of big pipeline projects between Canada and the United States. The United States is the world's largest producer of oil and gas. Biden's administration aims to transition the U.S. economy towards net-zero carbon emissions by 2050, and his initial moves towards that goal included cancelling a permit for the Keystone XL crude oil pipeline (KXL) and reducing oil-and-gas leasing.

The reaction from Biden's supporters, however, illustrates the challenge of managing the impact of the energy transition on different communities.

While climate activists celebrated KXL's demise, labor unions, reeling from the global oil downturn, have mobilized to keep ongoing projects from being derailed.

Mike Knisely, secretary and treasurer of the Ohio State Building and Construction Trades Council, which endorsed Biden, said he has been leaning on state officials to talk to the president about how his rapid-fire climate announcements are affecting his union membership's support.

"I tell them they need to get back with Biden and ask if this all really has to happen on Day Two of the new administration," Knisely said. "I just get so frustrated that there's almost no common ground (on pipelines) with the environmental community."

Climate groups have had successes in recent years, persuading large investors to reduce holdings in fossil fuel industries, as well as lobbying banks to shun investment in Arctic drilling.

But Biden was endorsed by a number of key labor unions that work on pipelines, refineries and other energy installations, including the International Teamsters and North America's Building Trades. Those unions celebrated the victory of a pro-labor president, but opposed the Keystone move, and are lining up against threats to the other pipelines.

Environmentalists see Biden as an ally in the battle to wean the United States off fossil fuels and stymie imports of carbon-intensive heavy crude from Canada's vast oil sands. They are intensifying efforts to shut three other pipelines: Enbridge Inc's Line 3 and Line 5, and Energy Transfer's Dakota Access Pipeline (DAPL).

Unlike KXL, these three lines are all currently in service. The Enbridge lines deliver crude oil and fuels from Canada, while DAPL sends crude from North Dakota to the Midwest and Gulf Coast.