Stephen Harper spoke on the Keystone XL project during a visit to New York City. The pipeline project carrying oil from Canada's tar sands would need approval from the U.S. State Department.
"The only real immediate environmental issue here is, do we want to increase the flow of oil from Canada by pipeline or via rail?" Harper said. He called rail "far more environmentally challenging."
Dozens of protesters from groups including the Sierra Club chanted against the pipeline as Harper arrived at the Council on Foreign Relations.
The Obama administration is considering whether to approve the pipeline, which would carry 800,000 barrels of oil a day from Alberta across six U.S. states to the Texas Gulf Coast. A decision is expected this summer.
The pipeline has become a flashpoint in the U.S. debate over climate change.
Republicans, and business and labor groups, have urged the Obama administration to approve the pipeline as a source of much-needed jobs and a step toward North American energy independence.
Environmental groups have been pressuring President Barack Obama to reject the pipeline, saying it would carry "dirty oil" that contributes to global warming. They also worry about a spill.
Obama's initial rejection of the pipeline last year went over badly in Canada, which relies on the U.S. for 97 percent of its energy exports.
The pipeline is critical to Canada, which needs infrastructure in place to export its growing oil sands production from northern Alberta. The region has the world's third largest oil reserves, with 170 billion barrels of proven reserves.
"I think all the facts are overwhelmingly on the side of approval of this," Harper said Thursday, adding that climate-affecting emissions from oil sands are "almost nothing globally."
TransCanada Corp. is now looking at a mid to late-2015 start-up for the pipeline project.
TransCanada spokesman Shawn Howard in a statement Thursday said the company appreciates Harper's support of the pipeline project and said Harper's advocacy "helps cut through the noise and misinformation about the project. He clearly understands the importance of this project for Canada."
Harper's wide-ranging comments also touched on Syria and Israel.
"There's nothing more shortsighted in Western capitals in our time than the softening of support we've seen for Israel around the globe," he said, calling the country "the one stable, democratic ally in this part of the world."
On Syria, Harper urged "extraordinary caution" on the idea of arming the opposition.
"We should not fool ourselves about what's happening in Syria," he said, saying there is "brutality and extremism on both sides."
"To start talking about arming unnamed people whose objectives we don't understand, I think, is extremely risky," Harper said.