Thousands of demonstrators encircled the White House on Sunday to protest the building of the Keystone XL pipeline that would carry crude oil from Alberta to Texas.

The protest came as U.S. President Barack Obama weighs the political pros and cons of the $7-billion project, which would create thousands of jobs and give the U.S. a safe source for oil. However, it has enraged environmentalists who warn a spill along the pipeline's route would contaminate drinking water.

TransCanada Corp. president Russell Girling, whose company wants to build the pipeline, said it makes "infinite sense" for the U.S. to go ahead with the plan.

"They consume at least 15 million barrels of oil a day, and they import 10 or 11 (million). The safest and most secure place to get that oil is Canada," he told CTV's Question Period.

A few famous faces turned out for Sunday's protest, including American actor Mark Ruffalo, 1997 Nobel Peace Prize winner Jody Williams, and Canadian actress Margot Kidder.

While police estimated the crowd of protesters hit 5,000 at the demonstration's peak, organizers said as many as 12,000 people were on hand for the human ring.

Protest organizer Bill McKibben called the ring around the White House "a big O-shaped hug," and "a symbolic house arrest."

While speaking to demonstrators on Sunday, Michael Brune, the Sierra Club's executive director, called on Obama to say no to the pipeline.

"President Obama can reignite the passion of Americans who care about clean air and clean water if he stands up for the health and livelihood of America's heartland, takes a stand against this climate catastrophe, and rejects this pipeline," Brune said.

"Denying the Keystone XL permit will send a clear signal that the U.S. government recognizes our true 'national interest' before oil company profits."

Obama, who is already in campaign mode as the U.S. nears an election year, said early last week he would give as much consideration to environmental issues as he would to job creation and energy security.

"Folks in Nebraska like all across the country aren't going to say to themselves, 'We'll take a few thousand jobs if it means our kids are potentially drinking water that would damage their health,"' Obama said in an interview with an Omaha TV station from the White House. "We don't want, for example, aquifers to be adversely affected. Folks in Nebraska obviously would be directly impacted."

When asked about Obama's comments while attending the G20 summit in Cannes, Prime Minister Stephen Harper said the president hasn't made a decision yet.

"I thought on balance they were noncommittal," he said of the comments. "It's a project that not only will create a vast number of jobs in both our countries but is essential to American energy security."

The pipeline is supposed to deliver 700,000 barrels a day of crude oil from the Alberta oil sands to refineries on the Gulf of Mexico. It will pass through Montana, South Dakota, Nebraska, Kansas, Oklahoma and Texas.

Girling said opponents of the pipeline have introduced "fear" into the debate and used celebrities like Ruffalo to get their message across to Americans. He said his company wants to make a "safe and reliable" pipeline that will not endanger anyone's water supply.

As for charges that oilsands development is contributing to global warming, Girling said the "Canadian oilsands are one tenth of one percent of global greenhouse-gas emissions."

Obama was expected to make his decision on the massive project by the new year, although a State Department spokesperson said last week that date was not set in stone.

"We'd like to get it done by the end of the year, but if thoroughness demands a little more time nobody has slammed the door on that," Victoria Nuland said. "Our first obligation to the American people, to the president, is to ensure that we do this in a rigorous, transparent and thorough way."

With files from The Canadian Press