A closely-watched gauge of future U.S. economic activity rose for the third straight month in May, showing the economy will remain robust through the spring and summer, a private research firm said Thursday.

The Conference Board (search) said its index of leading indicators rose 0.5 percent in May to 116.5, after a 0.1 percent gain in April.

The rise was slightly higher than expected on Wall Street, where analysts had anticipated a 0.4 percent rise.

"The data reflect a robust economic environment this spring and point to more of the same this summer," said Ken Goldstein, economist at the Conference Board.

"There are concerns about the high cost of gasoline, milk and a possible rise in short-term interest rates, but almost one million new jobs opened up in the last three months. Investment, inventory and even export growth continue to improve. This confluence of economic strengths is a recipe for continued job gains, and possibly a little more inflation," he added.

The Conference board said the pickup in the leading index was broad-based, with eight of 10 components rising in the month. The two negative contributors were consumer expectations and stock prices.

The index has climbed 2.0 percent in the six months since December, with nine of ten components gaining ground.

The board said its coincident index rose 0.3 percent in May, matching a gain the previous month. The lagging index advanced a milder 0.1 percent.