second-biggest insurer, said yesterday it will buy two annuities units of US-based Liberty Financial Cos. for $1.7 billion, following through on promises to pursue North American expansion aggressively.

The deal, the company's biggest yet, sees Sun Life pick up Keyport Life Insurance Co., a Boston-based provider of fixed and variable annuities, and New York-based Independent Financial Marketing Group, which markets annuities and mutual funds to small and mid-sized banks. The deal was announced early yesterday.

Sun Life, which has a presence in Bermuda, had been scanning the United States for potential annuities business but had not found the right price.

"When Liberty Financial came along, we knew it was right in our target area,'' Jim Prieur, president and chief operating officer at Sun Life, said in a telephone interview from Boston.

"Because we've been very disciplined about price, we've been unable to achieve success until this transaction.'' Sun Life is no stranger to the US market. It already owns Boston-based MFS Investment Management, which it said ranks as the No. 9 mutual fund firm in the United States, with $89 billion in assets under management.

Sun Life, which became a public company about a year ago, told shareholders at its annual general meeting last week that it wanted to expand its operations "aggressively'' around the world. The company said it would first look to expand its protection and wealth management businesses in North America, with plans to move into emerging markets in Asia later on.

With the acquisition of Keyport Life, Sun Life will gain nearly 300,000 policyholders and more than $17 billion in assets, giving it a stronger presence in the US life insurance market and increasing its assets under management by about 60 percent.