WASHINGTON — American manufacturing activity expanded in February at the fastest pace since 2004, though manufacturers have grown more concerned about rising inflation, according to a closely followed index released Tuesday.

The Institute for Supply Management said its manufacturing gauge rose to 61.4 percent in February from 60.8 percent in the prior month. Any reading over 50 indicates that more manufacturers are expanding instead of shrinking.

The manufacturing sector has now grown 19 straight months, outshining most other U.S. industries in that time. In 2010, the sector even gained jobs for the first time since 1997.

Economists surveyed by MarketWatch had forecast the index to edge up to 61 percent.

In a generally upbeat ISM report, however, companies flashed a warning sign about the cost of raw materials and inflation. The ISM’s prices index jumped to its highest level in more than two years. And some 66 percent of respondents said they paid more for raw materials; only 2 percent said they paid less.

Norbert Ore, who heads the ISM survey committee, said the price index “indicates significant inflation of raw materials costs across many commodities.”

The institute asks about 350 purchasing managers if their business got better or worse this past month. These senior executives are involved in all sorts of decisions, including hiring, the purchase of raw materials, the delivery of supplies, and the management of inventories.

Fourteen of 18 industries tracked by Tempe, Ariz.-based ISM expanded in February, spearheaded by apparel, oil and coal, and transportation.

The index for new orders, which is a strong indicator of future sales, rose to 68 percent from 67.8 percent. That was the highest level since 2004.

Production also increased, up to 66.3 percent from 63.5 percent the prior month.

The inventories index, meanwhile, fell to 48.8 percent from 52.4 percent, which is usually a sign of improved sales. Companies tend to draw down inventory in a recovery when demand for their products is higher than they expect.

What’s more, the backlog of orders rose to 59 percent from 58.0 percent. Backlogs tend to rise as demand increases.

The employment index, meanwhile, jumped to 64.5 percent from 61.7 percent in January — the highest reading since 1973. The employment index has only topped 60.0 percent three times in the past decade.