WASHINGTON — American officials said Wednesday that they had obtained credible information that at least some of the 48 Iranians captured in Damascus this month by Syrian rebels were members of Iran’s elite Islamic Revolutionary Guards Corps, not the pilgrims described by the government in Tehran.

The new information came as Obama administration officials stepped up accusations of broad Iranian assistance to the embattled government of President Bashar al-Assad of Syria.

Officials would not describe the details that indicated the Iranians held by the rebels of the Free Syrian Army were members of the Revolutionary Guards, citing the delicacy of intelligence-gathering and -sharing efforts in the region, so it was not possible to verify the reports independently. The Iranians were on a bus that they said was on its way to a Shiite shrine when it was seized by the rebels.

“We have received indications that at least some of the passengers on the bus are I.R.G.C., and now in F.S.A. custody,” said one American official, who discussed the information on the condition of anonymity.