MOSCOW, Aug 13 (Reuters) - Aluminium producer Rusal said on Friday that its financial performance in the second half of 2021 could be affected by Russia's new metal export taxes as well as the impact of the COVID-19 pandemic, despite strong results for January-June.

Hong-Kong listed Rusal, the world's third largest aluminium producer behind China's Chinalco and Hongqiao , on Friday reported a big jump in first-half net profit to $2.0 billion from a loss of $124 million a year ago due to higher sales and stronger aluminium prices.

But Rusal said it would not to pay dividends for January-June, sending its Moscow-listed shares down by 2.8%.

Rusal sells the metal, used in construction and electronics, to Europe, the United States and Asia. It has been hit by Russia's government decision to raise export taxes for the country's metals producers for August-December.

"The introduction of export duties on metals... as well as the persistence of pandemic uncertainties and the continued spread of new strains of the coronavirus infection... can significantly adjust the results of the second half of the year and the year as a whole," Rusal CEO Evgenii Nikitin, said in a statement.

Rusal said in June that the 15% export tax on aluminium exports, or a minimum of $254 a tonne, would cost it several hundred million dollars this year and may lead to lower 2021 sales.

Proceeds from a share buyback by metals miner Nornickel , in which Rusal owns a 27.8%-stake, supported its cash flow in the period. Rusal received $1.4 billion from its participation in Nornickel's programme, it said on Friday.

Rusal said it was due to repay $2.0 billion in loans by the end of 2022 and that its plan to modernise several aluminium smelters in Siberia along with the capacity expansion of the Taishet Anode Plant in Siberia would cost it $4.9 billion up to 2030. (Reporting by Polina Devitt; editing by Elaine Hardcastle and Jane Merriman)