US sen. asks health insurers about small businesses

* Harkin sends letters to four top companies

* Seeks details on premiums, compensation

* Industry group says prices mirror costs

By Susan Heavey

WASHINGTON, Nov 3 (Reuters) - Health insurance companies were asked on Tuesday for information on pricing for small businesses, the latest example of U.S. lawmakers putting pressure on the industry as Congress considers how to expand coverage to more Americans.

The chairman of the Senate health committee sent letters to Aetna Inc (AET.N: Quote, Profile, Research), Humana Inc (HUM.N: Quote, Profile, Research), UnitedHealth Group Inc (UNH.N: Quote, Profile, Research) and WellPoint (WLP.N: Quote, Profile, Research) following reports that small companies could expect a 15 percent rate hike next year.

"Health insurance companies should open their books and explain to the American people why they support a health insurance market for small businesses that is so dysfunctional, and so lacking in transparency," Senator Tom Harkin, an Iowa Democrat, said in a statement.

Harkin also asked for information from the insurers on officials making more than $5 million a year.

The move follows a number of other probes in the Senate and the U.S. House of Representative. Both chambers are pushing to finish legislation overhauling the nation's $2.5 trillion health care system by the end of the year, seeking to make good on one of President Barack Obama's top campaign promises.