CIA Director John Ratcliffe travelled to Russia on Tuesday for meetings in Moscow, US media reported.
Russia’s state-run Tass news service earlier reported that a large US military transport plane had landed in Moscow.
CBS News, CNN and The Wall Street Journal reported that Mr Ratcliffe had flown to Moscow. It was unclear if the meetings were about US efforts to end Russia’s war against Ukraine.
The CIA did not immediately respond to a request for comment.
The Journal said the US has collected intelligence showing Russia's plans for operations to test Nato's resolve.
The Boeing C-17 Globemaster III plane arrived at Vnukovo airport in the Russian capital in the morning after stopping in Latvia en route from Andrews Air Force Base near Washington, Tass reported, quoting online flight-tracking systems.
Russian President Vladimir Putin’s spokesman, Dmitry Peskov, declined to comment on the aircraft. The Kremlin has no meetings planned with US administration officials this week, he told reporters.
Anton Gerashchenko, a Ukrainian former deputy minister of internal affairs, said it was notable that the US had sent Mr Ratcliffe, as he is known to be hawkish on Russia.
"Given this, some view Ratcliffe’s visit as a warning that the US could harden its position if Russia continues to refuse to end the war in Ukraine," Mr Gerashchenko said on X.
He said the visit could be connected to Russia’s escalating hybrid war against Europe.
The US on Monday announced a sweeping new set of sanctions on Iran, telling countries to stop trading with Tehran or face economic retaliation.
Russia and Iran maintain close ties and Tehran has supplied Moscow with drones for its war against Ukraine.
In March, Ukrainian President Volodymyr Zelenskyy accused Russia of supplying its version of those drones to Iran.
A Ukrainian intelligence assessment reported by Reuters in April said that Russia has helped Iran with satellite imagery for use in attacking US bases in the Middle East.
Moscow denies providing such assistance and has said it is ready to help resolve the Iran conflict.
