FBI Director James Comey told a House Intelligence Committee hearing on Monday that the agency's investigation would include "the nature of any links between individuals associated with the Trump campaign and the Russian government" as well as any possible coordination between the campaign and Moscow's activities.
The congressional committee is examining allegations of Russia-linked meddling in the 2016 US election.
Comey told the panel he had taken the extraordinary step of confirming an ongoing investigation because of the extreme public interest in the case. He said the probe is part of the FBI's counter-intelligence mission, and would look at whether crimes were committed.
"Because it is an open, ongoing investigation and is classified, I cannot say more about what we are doing and whose conduct we are examining," Comey added.
The director of the National Security Agency (NSA), Mike Rogers, also testified before the panel. It is the first time the two intelligence chiefs have spoken publicly about their investigations into the possible links between Russia and associates of US President Donald Trump.
Russia denies that it attempted to influence the election.
Speaking later at a press briefing, White House spokesman Sean Spicer told reporters there was no evidence Trump's team colluded with Russia. "Investigating it and having proof of it are two different things," he said.
The committee was also examining controversial allegations by Trump that his Trump Tower in New York was wiretapped by former president Barack Obama's administration.
Trump first announced the unsubstantiated claim in several misspelled tweets on March 4.
FBI Director Comey told the hearing Monday there was "no information that supports those tweets," adding that it was outside the power of the president to order electronic surveillance of any US citizen.
The White House last week suggested Britain's GCHQ signals intelligence agency had helped Obama with the alleged surveillance - a charge the UK government dismissed as "utterly ridiculous." Rogers also rejected the accusation in the hearing, saying he had seen no evidence at the NSA to support it.
Relations with Russia
US intelligence agencies last year announced that hacking attacks against Democratic Party institutions were likely directed by the highest levels of the Russian government. The agencies said the hackers released embarrassing messages with the goal of helping Republican candidate Trump defeat his rival, Democratic candidate Hillary Clinton.
Both Rogers and Comey told the hearing that the intelligence community stands by that assessment.
The question of whether Trump and his campaign were coordinating with Russia has dogged his administration for weeks. Trump's first national security adviser, Michael Flynn, was fired last month after it emerged that he lied about contact with Russian officials before entering office.
New information surfaced last week that Flynn was paid $65,000 (60,350 euros) in 2015 by companies with ties to Russia.
Attorney General Jeff Sessions has also been accused of lying about his meetings with Russian officials.
nm/kms (Reuters, AFP, dpa)