Oil giant Royal Dutch Shell has issued a profit warning after it made less money than expected in the final quarter of 2013.

"Fourth-quarter 2013 figures... are expected to be significantly lower than recent levels of profitability," the company said in a statement.

It now expects profits for the quarter to be about $2.2bn (£1.3bn) and profits for 2013 as a whole to be $16.8bn.

Shell's shares fell more than 4% at the beginning of trading in London.

Profits for 2013 are expected to be down significantly on 2012, when it made $27.2bn.

"Our 2013 performance was not what I expect from Shell," Shell chief executive Ben van Beurden said in a statement.

"Our focus will be on improving Shell's financial results, achieving better capital efficiency and on continuing to strengthen our operational performance and project delivery."

Asset sales

Mr van Beurden became chief executive at the beginning of this year, taking over from Peter Voser.

Shell said its profits in the fourth quarter were hurt by a range of factors, including higher exploration costs, security problems in oil-rich Nigeria, and maintenance work that hit oil and gas production.

It said the weakening of the Australian dollar also had an effect.

Expected profits of $2.2bn for the quarter include impairments of $700m. When these are discounted, the expected profits are $2.9bn.

Shell's official annual results are scheduled to be published on 30 January.

It has seen a string of disappointing quarterly results over the past 12 months.

Earlier this week, it detailed plans to sell off some of its North Sea oilfield investments, and last year it began selling off some interests in the US shale gas industry.

In December, the Anglo-Dutch company also cancelled a $20bn project to build a gas processing plant in Louisiana.