Russia's President Vladimir Putin has thanked US counterpart Donald Trump for intelligence that helped foil "acts of terrorism" on Russian soil, according to a Kremlin statement.
Mr Putin and Mr Trump spoke on the phone on Sunday, it said.
The Kremlin said the information came via intelligence services, but it provided no further details.
Russian media is reporting the discovery of a plot to attack St Petersburg over the new year period.
Tass news agency says two Russian nationals have been arrested and plans to attack a mass gathering were seized, according to a spokesperson from the FSB, the Russian intelligence agency.
Mr Putin and Mr Trump have spoken on the phone and in person various times since the latter took office.
Records from the conversations show they have often talked about Syria, as well as nuclear agreements, North Korea and trade.
In December 2017, Mr Putin thanked Mr Trump for another warning from US intelligence agencies, which again apparently prevented a terrorist plot in St Petersburg, according to a White House account.
During that call, the Kremlin said Mr Putin had promised to reciprocate with information about terrorist threats to the United States.
Relations between Washington and Moscow plummeted after Russia annexed the Crimean peninsula from neighbouring Ukraine in 2014.
Ties were further strained when US intelligence agencies concluded that the Kremlin had interfered in the 2016 presidential election.
Despite this, President Trump and President Putin have appeared to be on good terms personally - and they have vowed to co-operate on terrorism.
Mr Trump has indicated he is considering attending the Victory Day celebrations in Moscow next May, after an invitation from the Russian leader.
Both countries have felt the threat of the Islamic State group.
In 2015, the group admitted downing a Russian passenger plane shortly after its take-off in Egypt - an attack that killed 224 people.
It is unknown whether those recently arrested in Russia have any links to a wider group.
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