Giant US insurer Liberty Mutual has registered a 27.3% slide in third-quarter earnings amid intense competition in US property and casualty lines.

Liberty Mutual is now losing money on its underwriting, with a combined operating ratio for the first nine months of the year of 100.1%. Its ratio for the third quarter alone fared little better at 98.7%.

The Boston-based insurer posted a net income of $US404 million ($441 million) in the third quarter, down from $US556 million ($607 million) over the same period last year, despite higher revenues of $US6.59 billion ($7.19 billion).

Net earned premiums for the quarter also rose 12% to $US5.79 billion ($6.33 billion).

More than half of Liberty's premiums are sourced outside the US.