Russia had nothing to do with alleged monitoring by hackers of internal email traffic at the US Treasury and Commerce departments, the Kremlin said on Monday.
People familiar with the matter told Reuters the hackers were believed to be working for Russia and that they feared the hacks uncovered so far may be the tip of the iceberg.
One person familiar with the matter said the hack was so serious it had led to a National Security Council meeting at the White House on Saturday.
Kremlin spokesman Dmitry Peskov dismissed the allegations.
"Once again, I can reject these accusations and once again I want to remind you that it was President (Vladimir) Putin who proposed that the American side agree and conclude agreements (with Russia) on cyber security," said Peskov, saying Washington had not responded to the offer.
"As for the rest, if there have been attacks for many months, and the Americans could not do anything about it, it is probably not worth immediately groundlessly blaming the Russians. We didn't have anything to do with it."