President Donald Trump has lashed out at the US intelligence community and media after new reports of contacts between members of his team and Russia.
Mr Trump accused the National Security Agency (NSA) and FBI of giving out information illegally.
Some US media say top Trump aides were in constant communication with Russian officials in the election campaign.
Intelligence officials previously said they believed Russia tried to influence the vote in favour of Mr Trump.
Moscow has dismissed the claim, describing it as based on unsubstantiated facts.
Kremlin spokesman Dmitry Peskov said on Wednesday that the latest reports of contacts with Trump aides were “not based on any facts”.
On Thursday Rex Tillerson is due for his first meeting as US secretary of state with Russian Foreign Minister Sergei Lavrov, at a G20 gathering in the German city of Bonn.
The latest allegations look set to reignite tensions between the president and his own intelligence services, which flared during the 2016 election campaign, says BBC Washington correspondent Gary O’Donoghue.
They come a day after National Security Adviser Michael Flynn resigned amid a row over reports he discussed US sanctions with Russia before Mr Trump took office.
Leading Republicans have joined calls for a wide investigation into Mr Flynn’s links with Russia.
Mr Trump appeared to attack the NSA and the FBI for what he described as leaks to the media about the contacts.
“Information is being illegally given to the failing @nytimes & @washingtonpost by the intelligence community (NSA and FBI?),” he tweeted.