May 12 (Reuters) - U.S. not-for-profit healthcare network Kaiser Permanente is planning to sell off large private-investment holdings due to cash constraints, the Wall Street Journal reported on Sunday.

Kaiser has been working with investment bank Jefferies Financial Group in recent months to sell up to $3.5 billion of private-fund stakes to secondary buyers, the Journal said, citing people involved with secondary deals.

Kaiser is expected to offer a similar sized collection later this year, according to the report.

Kaiser and Jefferies did not immediately respond to Reuters' request for comment.

Kaiser is one of the largest U.S. medical employers with over 24,000 doctors and 73,000 nurses. It operates with 40 hospitals and has over 618 medical offices, according to its website. (Reporting by Harshita Meenaktshi in Bengaluru; Editing by Josie Kao)