State Department spokesman Philip Crowley traced the origins of the surveillance programme to the 1998 bombings of US embassies in Kenya and Tanzania, arguing that the it had become customary procedure.
"We have acknowledged that we have a programme around the world where we are alert for people who may be surveilling our embassies because we recognize that they are potential targets of terrorism," he told reporters.
"We will be happy to answer any questions that any government has about the nature of the security measures to protect our embassies."
He spoke shortly after a top Swedish prosecutor announced he had opened a probe to determine whether the surveillance was illegal. Swedish Justice Minister Beatrice Ask earlier indicated the US embassy in Stockholm had secretly spied on Swedish residents in the capital since 2000.
Similar allegations surfaced in Oslo and Copenhagen last week, and in all three countries, national officials insisted they had not been informed of the surveillance activities, which extended to monitoring demonstrations and storing personal information about protesters.
If surveillance was carried out without authorization from the host countries, experts in the region insist it constitutes a violation of national laws.
But Crowley said "governments are fairly large, and there may be some instances where some agency of government has information and the other agency of government does not."
Norway's chief prosecutor has also asked police there to investigate the embassy's actions.
The US missions in Sweden and Norway have issued statements insisting their actions are part of common protection procedures.