New York's top financial regulator has expanded a probe intowhether reinsurance companies have written policies oninternational trade with Iran, which could potentially violate newU.S. sanctions.

In a letter posted to its website on Wednesday, the state'sDepartment of Financial Services asked reinsurers to explain theirdealings with entities and people that have ties to Iran.

The department also asked reinsurers to explain procedures inplace to ensure compliance with the Iran Freedom andCounter-Proliferation Act of 2012, which took effect on July 1.

Twenty reinsurers are getting the letter, including HannoverRe,Lloyd's of London and Swiss Re, a person familiar with the mattersaid.

Those reinsurers were among those contacted last month by theregulator, whose superintendent is Benjamin Lawsky, over theirdealings involving Iran.

Lawsky opened his probe after news reports thatSwitzerland-based Glencore Xstrata Plc and Trafigura AG hadsupplied thousands of tons of alumina to an Iranian firm thatprovided aluminium for Iran's nuclear program.

The new law bans financial services companies that do businessin the United States, such as insurers and reinsurers, fromproviding services to companies that trade with Iran.

Such a ban can make it harder for shippers to transact withIran, because they need insurance to protect against the risk oflosses on big shipments.