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O'Brien and other aides tried to quell controversy over what Trump knew and when he knew it, saying that a senior CIA civil servant decided not to brief Trump verbally "because she didn't have confidence in the intelligence."
He declined to say whether Trump got anything in writing.
"These are important allegations that, if they're verified I can guarantee you the president will take strong action. We've been working for several months on options," O'Brien told reporters outside the White House, though he added that Washington may never know the truth of the matter because of media leaks.
After Trump over the weekend said he was not briefed on the matter, the White House said Trump was not "personally" briefed but did not address whether he had received a written report, read it, and why he had not responded more aggressively if so.
The shifting statements have generated controversy among his fellow Republicans as well as Democrats and the suggestion that Trump may have ignored or not known about a threat to U.S. troops could damage him as he seeks re-election on Nov. 3.
U.S. and European investigators strongly suspect the unit of the GRU Russian military intelligence agency accused of targeting U.S. soldiers in Afghanistan using Taliban-linked militants is the same unit that was implicated in the poisoning of Russian intelligence defector Sergei Skripal in Britain, according to four sources familiar with intelligence reporting.
O'Brien on Wednesday said Trump has since been briefed on the Russian bounty reports, but declined to provide details.
