Aug 3 (Reuters) - U.S. biotech Amgen Inc (AMGN.O) on Tuesday said its second-quarter revenue rose 5%, but recovery from the COVID-19 pandemic, which has limited patient interactions with healthcare providers, is expected to hit sales for the rest of 2021. The pandemic "has suppressed the volume of new patients starting treatment," the company said in a statement. Amgen also said more competition, including from cheaper generics and biosimilars, would continue to pressure its net selling prices, especially for its cholesterol, psoriasis and infection-fighting drugs. In addition, the company said it is contesting in the U.S. Tax Court notices from the Internal Revenue Service seeking to increase its taxable income in three prior years by an amount that would result in additional federal tax of around $3.6 billion, plus interest. The dispute, which will take several years to resolve, involves Amgen"s accounting for profits between the United States and Puerto Rico, the location of most of its manufacturing operations, Chief Financial Officer Peter Griffith said on a conference call with investors. Amgen"s quarterly adjusted earnings, helped by share buybacks, rose 4% from a year earlier to $4.38 per share, beating the $4.10 forecast by Wall Street analysts, as calculated by Refinitiv. The results "look generally straightforward," but the commentary on the pandemic "will moderate enthusiasm," Jefferies analyst Michael Yee said in an emailed comment. Amgen shares, which rose 1.8% to close at $244.08, were trading lower at $242 after hours. Revenue of $6.5 billion was in line with analyst estimates as an 8% increase in unit sales volumes was partially offset by a 5% drop in net selling prices. For the full year, Amgen said it still expects adjusted earnings of $16.00 to $17.00 per share on revenue of $25.8 billion to $26.6 billion. But the company lowered its 2021 net earnings estimate to $8.84 to $9.90 per share from its prior view of $9.11 to $10.71, and said share repurchases will be at the upper end of its previous estimate of $3 billion to $5 billion. "This will raise eyebrows a bit given they typically raise guidance for the second half of the year," Yee said. Sales of newer migraine drug Aimovig fell 16% from a year earlier to $82 million, short of the $95 million projected by analysts. Sales of cholesterol fighter Repatha rose 43% to $286 million, but it too missed Wall Street estimates of $299 million. Amgen also announced on Tuesday plans to build a new drug substance manufacturing facility in Holly Springs, North Carolina.