US insurer Liberty Mutual Agency, a unit of the Liberty Mutual Group, is pursuing a $US1.41 billion ($1.5 billion) initial public offering (IPO) in a bid to slash debt.

The property and casualty insurer plans to sell 64.3 million class A shares in what would be the biggest US IPO so far this year.

Underwriters will also have the option of buying an additional 64.3 million shares.

Parent company Liberty Mutual Group plans to maintain a controlling interest in the subsidiary.