NEW YORK (AP) _ A key gauge of future economic activity rose 0.3 percentage points in May, more than reversing a 0.1 point decline in April and keeping the U.S. economy on track to achieve its longest expansion ever.

The Conference Board, a private business group, reported today that its Index of Leading Economic Indicators, which spots growth trends for the coming three to six months, rose to 107.4 in May from 107.1 in April.

``The leading indicators show the resilience and robustness of this expansion, which is poised to become the longest ever in early 2000,'' said Michael Boldin, director of business cycle research at The Conference Board. ``There is little evidence of cyclical imbalances that would jeopardize the economy's stability.''

The longest economic expansion in U.S. history lasted for 106 months, from February 1962 until December 1969. The current expansion, already the longest peacetime growth ever, would match the record if it continues through January 2000 and pass it if it lasts until February.

The Leading Indicators report came on a day Federal Reserve policy-makers were widely expected to raise short-term interest rates by one-quarter of a percentage point in a pre-emptive bid to cool the economy's torrid growth and prevent a rekindling of inflation.

But Boldin said Conference Board's report should not encourage Fed governors to make further rate hikes after today. He said even if the Fed were to raise rates a half point, it would not jeopardize the current pace of growth.

Nine out of 10 indicators that make up the leading index rose in May. The most significant increase came in a slowing of vendor deliveries. Slower deliveries means manufacturers are backed up in fulfilling their orders