A Tehran region security official told AFP on Wednesday tighter measures were in force, notably at the missions of Switzerland, Britain and Turkey.
"After the blasts in Turkey, we intensified the protective measures around some of the western embassies," said Ali Tala, in charge of security at the Tehran region governor-general's office.
A staff member at the Swiss embassy - which represents US interests in the absence of diplomatic relations between the United States and Iran - confirmed security there had been stepped up since Monday.
Tala was referring to last month's massive suicide bomb attacks in Istanbul, which killed a total of 62 people and left hundreds injured.
"We ordered an intensification of protective measures... and we will keep on holding discussions to confront any threats," the Tehran official said, adding that some of the information received on an al-Qaida threat came from media reports.
Istanbul was hit by two waves of deadly bombings last month, the first against Jewish synagogues, on November 15, and the second against British interests, on20 November.
Al-Qaida, led by the fugitive Saudi national Usama bin Ladin, has been blamed for numerous attacks, among them the ones on 11 September 2001 in the United States.