JERICHO, N.Y.—Aon Risk Services Inc. has agreed to buy brokerage Allied North America for an undisclosed sum.

Allied North America, which specializes in construction and surety business, is the 27th largest broker of U.S. business, based on $88.1 million in 2008 U.S. brokerage revenues, according to Business Insurance’s ranking.

“This acquisition reinforces our leadership position in the construction sector and enables us to bring additional talent and skills to the benefit of our clients,” Steve McGill, chairman and chief executive officer of Aon Risk Services, a unit of Chicago-based Aon Corp., said in a statement.

The deal is expected to close by year end.