A plunge in energy prices helped many Americans spend more freely in November and December, but it also weighed on those with livelihoods tied to the oil industry, the US Federal Reserve has said.

All told, the Fed's monthly Beige Book report on business activity showed the economy continued to expand, according to anecdotes collected from contacts nationwide on or before 5 January.

The report showed "most districts reporting a 'modest' or 'moderate' pace of growth".

Compiled by the Federal Reserve Bank of San Francisco, the report showed some consumers were getting a boost from lower gasoline prices, which helped holiday sales in the Chicago district and was leading people to buy bigger cars in the Atlanta district.

At the same time, growth in business activity in the Dallas district slowed, with several contacts expressing "concern about the effect of lower oil prices," it said. Texas is a major player in America's oil industry.

While output of energy-related products and services, such as oil well drilling, increased, "overall demand for energy-related products and services weakened somewhat," the report said.

Oil prices have declined sharply since June, reflecting in part slowing global economic growth and increased domestic production in the United States.