LOS ANGELES (Reuters) - Two US insurance companies said yesterday they will vigorously fight an anti-trust lawsuit brought against them and three other insurers by Ohio Attorney General Marc Dann.

The Hartford Financial Services Group Inc and Chubb Corp said they would defend themselves against charges of participating in a secret conspiracy with Marsh & McLennan Companies, Ace Limited and American International Group to fix prices and restrain competition for casualty insurance policies sold to Ohio businesses.

The lawsuit, filed last Friday in Cuyahoga County Court of Common Pleas, follows a three-year investigation by the Ohio Attorney General's Office and a settlement of similar allegations in New York, Illinois and Connecticut.

Hartford was "disappointed," spokesman Joshua King said in a statement, that Dann did not discuss the allegations with the company before filing suit, and intends to defend itself "vigorously."

The company "categorically denies the allegations" and cooperated fully throughout the Ohio investigation, he said.

Chubb spokesman Mark Greenberg said the insurer was disappointed to be included in the lawsuit after having cooperated fully in the Ohio investigation and regulatory probes in other states, and plans to vigorously contest the allegations.

The New York, Connecticut and Illinois probe concluded that Chubb did not participate in illegal bid rigging, as did an investigation by an independent outside law firm. Chubb contributed $15 million to a settlement fund because it had unknowingly benefited from the scheme, Greenberg said.

Marsh & McLennan said on Monday the company has already settled claims brought by most of its Ohio customers and intends to vigorously defend itself against the lawsuit.

Representatives for ACE and AIG had no comment on the lawsuit.