WASHINGTON, April 15 (Reuters) - The Export-Import Bank of the United States said on Thursday its board of directors had approved a year-long extension of its COVID-19 temporary relief measures through April 30, 2022, including for the Working Capital Guarantee Program and Supply Chain Financing Program. EXIM said the measures, first implemented in March 2020 and extended several times, had resulted in some $2 billion in working capital guarantee and supply chain financing authorizations. The government export credit agency said U.S. small business suppliers of larger exporters had seen significant reductions in payment times due to the measures, improving their cash flow. (Reporting by David Lawder Editing by Chris Reese)