CIA Director Bill Burns held a rare conversation with Russian President Vladimir Putin in Moscow last week, to convey “serious” US concerns about Russia’s military buildup along the Ukrainian border and to attempt to determine Russian intentions, two sources with direct knowledge told CNN.

Putin’s spokesperson also confirmed there was a discussion.

“Yes, such a conversation took place via telephone,” Putin’s spokesperson Dimitry Peskov told CNN on Monday

CNN reported Friday that Burns’ visit to the Russian capital took place at a time of growing concern among some US officials about the possibility of an expanded Russian military intervention in Ukraine.

One source told CNN that the US has “serious concerns” about the Russian build-up, adding, “It would be foolish for us not to be considering the possibility of an invasion or incursion.”