Top Pentagon officials pledged on Thursday to take action if the US military could corroborate intelligence suggesting Moscow paid militants linked to the Taliban to kill US soldiers in Afghanistan.
General Mark Milley, chairman of the Joint Chiefs of Staff, and Defence Secretary Mark Esper spoke before a congressional committee as the Trump administration comes under pressure to explain media reports claiming the president was briefed on the intelligence but did nothing in response.
Gen Milley said the information was "not corroborated".
"We'll get to the bottom of it. We are going to find out if, in fact, it's true. And if it is true, we will take action," he said, without specifying what kind of action might be taken.
Washington has known for years that Russia has been supporting the Afghan insurgents, including through arms shipments, Gen Milley said.
But, in the case of the Russians, "we do not have concrete corroborating evidence, intelligence to show directing. That's a big difference," he said.
"All the defence intelligence agencies have been unable to corroborate that report," Mr Esper agreed.
Even so, he added: "The commanders take all reports seriously, regardless of the degree of credibility or confidence."
The New York Times, followed by other US media, first reported on the intelligence last month.
The White House has said President Donald Trump was not briefed on the intelligence because it was unverified.
But the Times, citing a number of officials, reported that the intelligence was included in the written presidential daily brief in February.
The newspaper reported that US intelligence officers and special forces in Afghanistan began raising the alarm as early as January, and that the National Security Council held an inter-agency meeting in late March to discuss possible responses – but the White House did not authorise any action.
Russia and the Taliban have denied the claims.
The scandal comes with Mr Trump trying to withdraw troops from conflict-torn Afghanistan – one of the Taliban's key demands – and end America's longest war.
