The US is not ready to collaborate militarily with Russia, defence secretary Jim Mattis said, appearing to close the door for now on any effort to work more closely with Moscow in the fight against Islamic State militants in Syria.
His blunt rejection came after Russian President Vladimir Putin called for increased intelligence co-operation with the US and Nato, and it makes such co-ordination less likely, at least in the near future.
Mr Mattis followed his dismissal with a sharp assessment of Russia's alleged election meddling, saying there is "very little doubt that they have either interfered or they have attempted to interfere in a number of elections in the democracies".
His comments raised questions about the Trump administration's policies on Russia.
As a candidate, President Donald Trump repeatedly praised Mr Putin, saying he wanted a new era of co-operation with Moscow.
Speaking at a meeting of Nato defence ministers, Mr Mattis said the US will continue to engage politically with Mr Putin's government to try to find common ground.
Political leaders, Mr Mattis said, will seek "a way forward where Russia, living up to its commitments, will return to a partnership of sorts here with Nato. But Russia is going to have to prove itself first".
The US ceased military-to-military relations with Russia in the wake of Moscow's 2014 annexation of Ukraine's Crimea region.
But last year, the Obama administration considered plans to co-operate militarily with Russia as part of a ceasefire deal in Syria.
Senior defence department leaders opposed the plan, and it quickly fell apart as the ceasefire collapsed.
Also on Thursday, US secretary of state Rex Tillerson met with Russian foreign minister Sergey Lavrov in the highest-level face-to-face contact between the two countries since Mr Trump took office.
Mr Lavrov was asked if Russia is concerned about turmoil in the Trump administration.
He repeated Moscow's standard line that Russia "does not interfere in the domestic matters of other countries".
Mr Tillerson did not speak at the meeting on the sidelines of a conference of foreign ministers of Group of 20 major powers in Bonn, Germany.