Oil started to flow on Wednesday through the southern US leg of the controversial Keystone XL pipeline, its Canadian operator said, while Washington mulls whether on not to approve its northern half.
Calgary-based firm TransCanada said the US$2.3 billion pipeline had begun carrying crude 487 miles from Cushing in Oklahoma to Gulf Coast refineries in the southern state of Texas.
The company projected that the pipeline's capacity for the first year of operation would stand at 520,000 barrels per day, but said that could be scaled up to 700,000 barrels.
The Keystone XL project was first proposed back in 2008, but after years of delays, TransCanada split it in two, and construction began on a southern section that did not require US presidential approval.
President Barack Obama is to decide this year whether to green light the remaining 1,179-mile northern leg from Alberta in Canada, south through environmentally sensitive parts of the United States.
Opponents say extracting oil from Alberta's oil sands produces more of the carbon emissions that cause climate change than standard drilling, as it must be steamed and separated from the sands.
Industry groups and Republican US lawmakers have promoted the project as a job creator and Canada bills itself as a safer source of energy for the United States than oil producers in the Middle East.