Fosun International Ltd, an investment company controlled by Chinese billionaire Guo Guangchang, said it had offered to buy the 80 percent it does not already own in U.S. insurer Ironshore Inc for $1.8 billion.

The move follows Fosun's decision to buy a 20 percent stake in the casualty and property insurer for $464 million in August 2014 in a bid to expand its global insurance network.

"This acquisition will bring synergies for both parties in the prevention of currency risks, expansion of assets allocation and cooperation in reinsurance business," Fosun said in an announcement to the Hong Kong stock exchange.

Fosun has identified insurance has a major growth area for the group. Last year it bought the insurance arm of Portuguese state bank Caixa Geral de Depositos SA for 1 billion euros ($1.1 billion).

Ironshore had total assets of $6.7 billion at end-December 2014, although net profit after tax fell 13.3 percent to $84.5 million from a year ago, the Fosun statement said.

Gou's conglomerate is emerging as one of China's most acquisitive and aggressive private-sector companies as it buys a wide range of global assets.

Fosun has launched about $6 billion worth of deals in 2014 and in 2015, excluding this latest deal.

Moody's Investors Service has estimated that these investments have largely been funded with debt, increasing its gross debt by around 40 percent.

Nevertheless, Moody's upgraded Fosun's senior unsecured debt rating, citing sizeable cash and liquid assets, and said it expected Fosun to continue to "engage in aggressive investments".

Deals so far include the purchase of French resorts operator Club Mediterranee in a bitterly contested takeover battle.

The Ironshore deal is subject to regulatory approval, and both parties could call off the transaction if it is not consummated by March 31, 2016, the statement added.

Fosun was founded in 1992, and its core businesses comprise insurance, steel, property, pharmaceuticals and mining.