Congress Working Committee will hold a meeting soon to review the results but it goes without saying that we as a party collective must draw the appropriate lessons from this setback in a spirit of humility and honesty," she added.
Sonia Gandhi also congratulated Tamil Nadu chief minister MK Stalin and West Bengal chief minister Mamata Banerjee.
Earlier on 2 May, the Congress had said that the party would "study" the results, correct mistakes and do a course correction.
Congress spokesperson Randeep Surjewala read out a statement on the same day saying, "The Congress will definitely study the results and all the reasons diligently and we are committed to correct our mistakes and do appropriate course correction."
"We recognise that the election results are not as per our expectations, particularly those of Assam and Kerala," he added.
Surjewala said the people's mandate is the final word in a democracy. "People of West Bengal, Assam, Kerala, Tamil Nadu and Puducherry have given their democratic mandate for the next five years. We accept the verdict with humility and a sense of responsibility."