UnitedHealth CEO Stephen Hemsley is warning analysts not to assume that the insurer will participate widely in a key health care overhaul coverage expansion that unfolds later this year.

State-based exchanges will debut this fall and allow people to sign up for coverage online, with some using income-based tax credits to help pay the bill. Hemsley says UnitedHealth may participate in as few as 10 exchanges when as many as 100 might be set up.

He says whether the insurer participates will depend on if the exchanges are fair and provide a reasonable financial return.

Analysts say it's too early to determine whether some exchanges will wind up thin on competition. Competition on exchanges is intended to keep prices in check.

UnitedHealth is the nation's largest health insurer.