Stephen Schwarzman, the Blackstone Group co-founder, chair and chief executive, warned Monday that an inflating bubble in asset values could lead to a painful reckoning, particularly for late-stage investors in private technology companies like WeWork parent We Co.

Private investment valuations are getting very high, Schwarzman said in an interview, adding that later-stage funding rounds in companies planning initial public offerings were helping to escalate those values, even for companies that often have little prospect of profitability.