Congress president Sonia Gandhi on Tuesday expressed her grief over the death of former Nepal prime minister Sushil Koirala, saying he "led his country during extremely difficult and testing times"."He led his country during extremely difficult and testing times and contributed immensely to the transition of Nepal into a Republic," she said in a statement."Underlining the close relation with Nepal as also that with the Nepali Congress, to which Koirala belonged, his contribution to fostering Indo-Nepal ties will always be remembered," Sonia added.Sushil Koirala, former prime minister and president of the Nepali Congress (NC), died of pneumonia early on Tuesday. He was 80.