It could take months for Nexen to fully understand what caused a pipeline leak that spilled five million litres of bitumen and water into muskeg near Fort McMurray, Alta., the company says.

Nexen CEO Fang Zhi, who spoke to the media Wednesday at the site of the spill, personally apologized for the damage.

"It's disheartening to see the site here, and it is very disappointing that this has happened," said Fang, who took over as CEO of Nexen last April, about a year after the company was bought by China's state-controlled CNOOC Ltd.

The pipeline that is now the focus of an investigation and cleanup efforts was shut down for much of June, so it could be cleaned, said Ron Bailey, vice-president of Nexen's Canada operations.

Hot water was run through the pipeline, and it resumed operations on June 29. The leak was discovered on July 15 by a worker walking in the area.

The leak spilled about 1.6 million litres of bitumen and 3.4 million litres of water, Bailey told reporters at the site.

The bitumen is being left to cool and solidify, he said, because it poses no extra environmental hazard in that state.

The most immediate focus, he said, is on the spilled water, which contains chlorides.

Bailey said the highest concentration of chlorides found in test water samples at the site has so far been 2,000 parts per million. Potable water has 500 parts per million by comparison, he said.

Up to 130 people are working on the cleanup each day, Bailey said. The company has set up six-foot game fences around the spill along with two-foot fences to keep out amphibians and small animals. Cannons are being used to scare off birds.

The company still has no clear idea exactly when the leak began or what caused it, or how long the cleanup might take, Bailey said.

"This is going to take us some significant time to understand exactly what has happened here. When I say significant time, we're talking months."

The pipeline had only being operating for eight months before the spill. It is what's called a pipe-in-pipe design, and investigators can so far only see the exterior pipe.

The spill covers about 16,000 square metres, an area approximately the size of two CFL football fields. It is mostly contained within the pipeline corridor.

The area can only be reached by a winter access road, so the company had to build a road into the site. Vacuuming of the oil started last Friday. The spill has been contained by berms and other abatement equipment.

Alberta's Environment Minister Shannon Phillips and Energy Minister Marg McCuaig-Boyd plan to visit the site Friday.

"Our focus as of now is ensuring the safety of our workers on the site, minimizing whatever impact on the environment and the wildlife, as well as understanding to root causes of this incident through investigations," Fang said.

He was asked about the possible long-term political fallout, given that oil producers in Canada want to build more pipelines to take their products to foreign markets.

He said it was far too early to talk about that.

"This industry grows by sharing and taking lessons from incidents that have occurred in the past," he said.