June 21 (Reuters) - The U.S. Food and Drug Administration on Thursday approved Roche's drug for a chronic blood disorder, the Swiss drugmaker said.

The drug crovalimab, branded as PiaSky, is a monthly under-the-skin or intravenous treatment for paroxysmal nocturnal hemoglobinuria (PNH).

PNH is a disorder in which red blood cells break apart prematurely. It can cause anemia, fatigue and blood clots, and can lead to kidney disease.

Roche said the disease affects around 20,000 people worldwide.

The approval is based on a late-stage study in which PiaSky showed a 79.3% control in the destruction of red blood cells versus 79% for the standard-of-care eculizumab from week 5 to week 25.

"Crovalimab could provide an option to self-administer as infrequently as every four weeks, thereby reducing clinic visits for people with this lifelong condition," said Levi Garraway, chief medical officer of Roche.

Other treatments for PNH such as Astrazeneca's Ultomiris and eculizumab, sold as Soliris, and Amgen's Bkemv require infusion by healthcare professionals.

PiaSky was approved in China in February and Japan in March.

The drug is also being tested in two other blood disorders, atypical hemolytic uremic syndrome and sickle cell disease, and a kidney disease called lupus nephritis. (Reporting by Puyaan Singh in Bengaluru; Editing by Vijay Kishore)