That assessment, the source said, was backed up by "several pieces of information" that supported the view that there was an effort by the Russian intelligence unit -- the GRU -- to pay bounties to kill US soldiers, including interrogation of Taliban detainees and electronic eavesdropping. The source said there was some other information that did not corroborate this view but said, nonetheless, '"This was a big deal. When it's about US troops you go after it 100%, with everything you got."
Trump is not known to fully or regularly read the President's Daily Brief, something that is well-known within the White House.
The information was serious enough the National Security Council staff held a meeting during the spring to discuss "possible response options," including sanctions, if the intelligence developed to the point it was deemed ready to take to the President for any possible action, the official said.
Pressed on Monday whether the information was included in the President's Daily Brief -- the written document that includes the intelligence community's more important and urgent information -- White House press secretary Kayleigh McEnany said only that Trump "was not personally briefed."
The US intelligence community and military are investigating to see if there is further corroboration, and if the National Security Council staff can recommend response options. The source said the White House is now claiming the leak will ruin the opportunity to get a real answer.
There is "no consensus within the intelligence community" about whether Russia offered to pay bounties to the Taliban for killing American troops, McEnany said. There is "dissent" within the intelligence community about the intelligence, which she insisted had not reached Trump's desk because "it wasn't verified."
The source tells CNN that intelligence of this nature with risk to US troops should be assumed to be true until you know otherwise.
The cascade of developments around the Russian effort have prompted a bipartisan group of congressional leaders to demand that the Trump administration explain what it knew and when.
The White House briefed a group of House Republican lawmakers on the matter on Monday, while Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer called on Director of National Intelligence John Ratcliffe and CIA Director Gina Haspel to provide all-member briefings to Congress on the intelligence.
And several key Senate Republicans said they are seeking more information from the Trump administration, too.
"It seems clear that the intelligence is real. The question is whether the President was briefed," Pelosi told CNN's Jake Tapper on Monday. "If he was not briefed, why would he not be briefed? Were they afraid to approach him on the subject of Russia?"
This is a breaking story and will be updated.
CNN's Kaitlan Collins contributed to this report.