US insurer Chubb says second-quarter net profit fell 1% to $US494 million ($679.2 million), with higher catastrophe losses partly offset by premium growth in commercial and personal lines.

Net written premium gained 3% to $US3.3 billion ($4.53 billion), with commercial lines up 5% and personal lines rising 2%.

The combined operating ratio improved to 85.5% compared with 90% in the corresponding quarter last year.

Catastrophe losses totalled $US148 million ($203.43 million) before tax, up from $US146 million ($200.68 million).

At the end of the second quarter Chubb struck a deal for Zurich-based insurer Ace to acquire it for $US28.3 billion ($38.9 billion), creating the second-largest commercial insurer in the US.

The transaction, expected to close in the first quarter of next year, is subject to shareholder and regulatory approval. Chubb will not comment on earnings results until the deal is complete.