Clapper, who has worked in intelligence under every US leader from John F Kennedy to Barack Obama, said his professional instincts had always been loyalty to the president, regardless of which party they come from.
But for Trump he is making an exception.
"Now as a private citizen, I am very concerned about the assault on our institutions coming from both an external source -- read Russia -- and an internal source, the president himself," he said in Australia.
The US administration is at the centre of an ever- widening probe into Moscow's meddling in last year's election and possible collusion by the Trump team.
Clapper said he was at a loss to know why Trump's team appeared so keen to court Moscow.
"I characterise it as inexplicable. I don't understand that," said the director of national intelligence under Obama.
"Is there a smoking gun with all the smoke? I don't know the answer to that."
Sacked FBI chief James Comey is set to testify Thursday as the star witness in the Senate Intelligence Committee's investigation of alleged interference and whether Trump pressured him to halt a probe into an advisor's links to Russia.
He is expected to dispute the president's claim that he told the former reality television star multiple times that he was not under investigation by the FBI.
Clapper said the "inexcusable manner" in which Comey was dismissed by Trump reflected "complete disregard for the independence and autonomy of the Federal Bureau of Investigation, our premiere law enforcement organisation".
"I worry about these assaults on our institutions."
He characterised Moscow's campaign to interfere with the US election as "unprecedented in its directness and aggressiveness".
"Social media trolls planting false information, orchestrated fake news which many other news outlets picked up and amplified either wittingly or unwittingly," he said.
Disclaimer: No Business Standard Journalist was involved in creation of this content
You’ve reached your limit of {{free_limit}} free articles this month.
Subscribe now for unlimited access.
Already subscribed? Log in
Subscribe to read the full story →
Smart Quarterly
₹900
3 Months
₹300/Month
Smart Essential
₹2,700
1 Year
₹225/Month
Super Saver
₹3,900
2 Years
₹162/Month
Renews automatically, cancel anytime
Here’s what’s included in our digital subscription plans
Exclusive premium stories online
Over 30 premium stories daily, handpicked by our editors


Complimentary Access to The New York Times
News, Games, Cooking, Audio, Wirecutter & The Athletic
Business Standard Epaper
Digital replica of our daily newspaper — with options to read, save, and share


Curated Newsletters
Insights on markets, finance, politics, tech, and more delivered to your inbox
Market Analysis & Investment Insights
In-depth market analysis & insights with access to The Smart Investor


Archives
Repository of articles and publications dating back to 1997
Ad-free Reading
Uninterrupted reading experience with no advertisements


Seamless Access Across All Devices
Access Business Standard across devices — mobile, tablet, or PC, via web or app
