May 11 (Reuters) - FS KKR Capital FSK reported a wider first-quarter loss and a sharp drop in net asset value, hit by portfolio losses, new non-accruals and spread widening, and launched a $300 million share repurchase program.

Net asset value per share fell to $18.83 as of March 31 from $20.89 at the end of the prior quarter, while loss per share widened to $1.57 from 41 cents.

Total investment income declined to $304 million from $400 million a year earlier, reflecting lower interest and fee income, while total net realized and unrealized losses rose to $558 million from $67 million.

FSK said the decline in net asset value was driven by investments that had weighed on prior quarters, new non-accrual assets and market-driven spread widening in parts of its portfolio. Non-accrual investments rose to 4.2% of the portfolio at fair value from 3.4% at Dec. 31, while the investment portfolio fell to $12.27 billion at fair value from $13.01 billion at year-end.

Purchases totaled $499 million in the quarter, compared with $710 million of sales and repayments. Net debt-to-equity rose to 131% from 122%, while the weighted-average yield on accruing debt investments slipped to 9.9% from 10.1%.