December's purchasing managers index dropped slightly from November's 10-month high, dropping from 54.7 to 54.5 in the month, Markit said.
The index indicates growth with numbers above 50. The flash estimate, however, is subject to revision as more data becomes available.
Markit said the new orders index slid from 56.2 to 54.5. The new export orders index held steady at 51.4.
The employment index rose from 52.3 to 53.7. The backlog of work index indicated slightly higher growth at 52.6, up from November's 52.
"The flash PMI remained surprisingly high in December, suggesting strong growth momentum in the goods producing sector," Markit Chief Economist Chris Williamson said in a statement.
Williamson said the U.S. economy had held up surprisingly well considering the "headwinds of the uncertainty caused by the government shutdown and debt crisis."
The recovery's resilience "will add to the likelihood of the Fed [U.S. Federal Reserve] tapering its huge stimulus of $85 billion per month asset purchases sooner rather than later," he said.