Norway's $700 billion oil fund made its first property purchase in the United States on Monday and plans to spend billions more this year enlarging its portfolio, its real estate chief said.

The fund, the world's biggest sovereign wealth fund, bought stakes in five properties for around $600 million from U.S. pension fund TIAA-CREF and is looking for more assets, it said in a statement.

The deal is part of the fund's commitment to diversify, gradually moving its focus away from Europe and widening the types of assets it holds as it invests Norway's surplus oil income.

"Having roughly one-third of the portfolio in the U.S. is a good target. Of course, that could mean 25 percent or 40 percent and we can debate that over time," Karsten Kallevig, the fund's real estate chief told Reuters.

The fund can hold 5 percent of its portfolio in real estate, or around $35 billion, indicating that it could build a U.S. portfolio in excess of $11.5 billion.

Its properties are still worth less than 1 percent of the fund so it has close to $30 billion left to spend before reaching its limit.

The fund is initially looking for low risk transactions as building the portfolio puts a burden on its management capacity.

"We are in the build-up phase so it makes sense to probably start out with more core assets and not move too fast up the risk curve too early," Kallevig said.

"We have started at the lower risk end of the spectrum and higher risk very often means more complex, more difficult transactions," he said.

On Monday it purchased stakes in five high-end commercial properties in places including New York's Park Avenue and Washington's Pennsylvania Avenue, and said it may look for more deals with TIAA-CREF, a retirement provider for academics, researchers and others.

Diversification, into areas such as logistics, was only a matter of time, it said.

There's nothing in my mandate that says you can only do 'boring' transactions," Kallevig said.

The fund will spend this year focusing on Europe and the major east coast cities of the U.S., then will turn its attention to Asia and the rest of the Americas "sometime in the future."

"If you were a betting guy, you'd look at the biggest and most liquid markets. In Asia, there are 4-6 countries that are meaningful... Japan, Australia, Hong Kong, Singapore, China and Korea," Kallevig said.

"In the Americas, there's Canada, Mexico, Brazil and Argentina," he said, adding that this was a list of places for potential investment and not a commitment.

The fund, managed by Norway's central bank, started buying property in 2011 with high end property in Paris and London. It made a $1.6 billion investment into commercial property in December.

It generally buys property in partnership with a more experienced property investor or alone, if the property has a long-term secured tenant.