By Ben Dummett and Judy McKinnon 

Talisman Energy Inc. lost $1 billion in the fourth quarter after recording impairment charges totaling more than $800 million on its underperforming, high-cost assets in the U.K. and Norway, and the company said it would continue to sell operations to shore up its balance sheet.

Calgary, Alberta-based Talisman and other resource producers, under pressure from shareholders, are shedding less-profitable assets and focusing on higher-quality assets to boost earnings and stock prices.

"Our objective is to create sustainable value for our shareholders, and we will continue to position the company to achieve this by generating near-term steady cash flow from our best assets in our two core regions" of North America and the Asia Pacific, Talisman Chief Executive Hal Kvisle said in a statement.

In December, Talisman named two nominees of U.S. activist investor Carl Icahn to its board. That came less than two months after Mr. Icahn disclosed taking a stake in the oil and gas company and indicated he might push management to consider strategic alternatives and add new directors.

Talisman has posted a string of disappointing financial results, hurt in part by an extended gas-price slump. In an effort to turn around its operations, it has announced deals to sell more than $2.2 billion of noncore energy and pipeline operations. That is more than two-thirds of the $3 billion of total asset sales the company had targeted.

The company said Wednesday the target rose to $4.2 billion in noncore assets sales over the next 12 to 18 months. Assets now on the block include its Norwegian oil and gas business in the North Sea and its Duvernay oil and gas exploration play in Alberta, a company spokesman said.

Talisman lost $1.01 billion, or 98 cents a share, in the fourth quarter, compared with a profit of $376 million, or 37 cents, a year earlier. Adjusted to exclude items, it said it lost 11 cents a share, or well below the break-even results analysts polled by Thomson Reuters expected.

One of the biggest items that contributed to the latest results was a $277 million charge for lower reserve estimates and higher operating costs at its joint-venture U.K. North Sea oil and gas operation. The company now also anticipates higher costs when it decommissions the U.K. drilling platform. It also took a charge on lower reserve revisions and higher costs at its Norwegian North Sea operation.

Cash flow also dipped to $580 million from $675 million. Production fell to 387,000 barrels of oil equivalent a day from 392,000 barrels a day a year earlier, hurt by a 33% drop in output from the North Sea due to planned and unplanned outages.

Talisman said its net debt at the end of 2013 was about $4.8 billion, up from about $3.7 billion a year earlier.

Talisman, which relies on natural gas for the bulk of its earnings, said it expects the Americas and Asia Pacific to contribute more than 90% of its production this year.

Write to Judy McKinnon at judy.mckinnon@wsj.com

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