FBI Warns Donald Trump and Hillary Clinton Staffers to Beware of Foreign Spies in US Warnings come amid increasingly aggressive espionage efforts from abroad.

 -- Be careful when you're meeting new people in the nation's capital and elsewhere, because you could be a prime target for foreign spies.

That's the warning FBI agents gave to presidential campaign staffers Wednesday during two separate security briefings in Washington, D.C., according to sources.

While the briefings were portrayed as routine, they come amid increasingly aggressive efforts by foreign governments to access U.S. secrets and potentially influence upcoming elections here.

Wednesday’s unclassified briefings were held just blocks from the White House, in the building shared by rival transition teams for Donald Trump and Hillary Clinton, the sources said.

An FBI official described the sessions as voluntary "awareness briefings" provided to "low-level staffers" of the two major-party campaigns every presidential election cycle.

Each briefing Wednesday – a morning session for the Trump team and a later one for the Clinton team – lasted about an hour, ABC News was told.

One campaign source said FBI agents laid out a "standard best practices briefing," emphasizing it was "not precipitated by any particular [or known] threat."

Nevertheless, the FBI official acknowledged the unique threat environment facing the United States, particularly espionage efforts from Russia.

Officials acknowledged earlier this week that suspected Russian hackers successfully stole voter-related information from the election system in Illinois. Around the same time, "a known Russian hacker" instigated a cyberattack on Arizona’s election systems, but attempts to access the system failed, according to a spokesman for the secretary of state’s office in Arizona.

Meanwhile, the FBI is still investigating what appear to be coordinated cyberattacks on the Democratic National Committee and other Democratic organizations. Authorities suspect the computer-based assaults were likely launched out of Russia, according to sources.

The nation’s top intelligence official recently said it’s "not surprising" to see Russia trying to undermine the U.S. political process, especially because Russian President Vladimir Putin is "paranoid" about "the potential for a 'color revolution' to occur in Russia."

"They believe we're trying to influence political developments in Russia, we're trying to effect change, and so their natural response is to retaliate and do unto us as they think we've done to them," Director of National Intelligence James Clapper said in July at the annual Aspen Security Forum in Aspen, Colorado.

The last time two non-incumbents were major-party nominees for the White House – in 2008 – foreign spies engaged in espionage activity "like no other" presidential campaign before it, according to a document from Clapper's office cited by the Associated Press.

Leading up to that election, foreign spies "met with campaign contacts and staff, used human source networks for policy insights, exploited technology to get otherwise sensitive data, engaged in perception management to influence policy," said the document, which was used in a slide show warning new Obama administration officials that they could be targeted by foreign spies.

Wednesday’s sessions likely mirrored briefings that the FBI routinely gives to staffers working in the U.S. Senate and House of Representatives, and to others working in government-related affairs across the country.