UPDATE 1-Shell oil sands costs rise again, partner says

* Athabasca oil sands expansion's costs rise to $14.3 bln

* Cost estimate up 4.4 pct over past year (Adds comment from Shell spokesman in 5th paragraph. In U.S. dollars unless noted)

CALGARY, Alberta Feb 25 (Reuters) - The cost of a 100,000-barrel-per-day expansion of Royal Dutch Shell Plc's (RDSa.L) Athabasca oil sands project has climbed to $14.3 billion, Chevron Corp (CVX.N), one of its partners, said in a filing.

The new estimate amounts to $600 million more than the estimate provided by Chevron a year earlier.

Chevron, which holds a 20 percent stake in the oil sands mining and upgrading project, said the expansion will boost output to 255,000 barrels per day.

The cost of completing the project has steadily climbed well beyond Shell's 2006 estimate of between C$10 billion and C$12.8 billion ($9.4 billion to $12 billion). Just a year ago, Chevron pegged the cost of the project at $13.7 billion.

Shell spokesman Philip Vircoe said much of the rise in the Chevron estimate came as the Canadian dollar gained on its U.S. counterpart. "The estimate is not new to us."

Over the years, cost overruns have been widespread for the massive projects needed to tap the oil sands, the largest crude reserves outside the Middle East.

However, rival producers in the region said costs have fallen during the past year, mostly because of reduced labor costs. As most projects were rejigged, delayed or canceled because of the financial crisis, the squeeze on a limited pool of skilled labor has eased.

Chevron's filing did not say why it had boosted its cost estimate for the project.

Shell owns 60 percent of Athabasca, with Chevron and Marathon Oil Corp (MRO.N) each holding 20 percent. The project includes an oil sands mine near Fort McMurray, Alberta, and an upgrading refinery near Edmonton. ($1=$1.06 Canadian) (Reporting by Scott Haggett; Editing by Himani Sarkar)