Russian President Vladimir Putin thanked his US counterpart Donald Trump on Sunday for the CIA's help in thwarting a planned attack in Saint Petersburg, the second time in a week that the leaders have exchanged praise.

Putin spoke by phone with Trump to convey his gratitude for intelligence supplied by the CIA which allowed Russia's FSB security service to break up a "terrorist cell" that was planning attacks in Russia's second city, according to the Kremlin.

"The information received by the CIA was enough to detect, hunt down and arrest the criminals," it added in a statement carried by Russian news agencies.

Putin also pledged that Russian security agencies would pass on any information received about terrorist threats to the United States and its citizens.

The White House said the foiled attack could have killed "large numbers of people."

It stressed that the cooperation "serves as an example of the positive things that can occur when our countries work together."

The FSB announced on Friday it had arrested seven members of an Islamic State group cell that had been planning a suicide bombing and "the killing of citizens" in crowded areas of Saint Petersburg on December 16.

- On alert ahead of World Cup -

Police confiscated a large number of explosives used to make homemade bombs, automatic rifles, munitions and extremist literature, it said.

On Tuesday, FSB chief Alexander Bortnikov said Russia was on alert for the possible return of jihadists from Syria ahead of the World Cup and the presidential election in 2018.

Russia has suffered several attacks this year, including a bombing on the Saint Petersburg metro in April that left 14 people dead.

The threat of attack has increased since Moscow's military intervention in Syria in September 2015 to support President Bashar al-Assad's regime, making Russia a priority IS target.

As many as 40,000 fighters travelled from all over the world, including Russia, to join IS in Syria after the 2014 declaration of its self-styled "caliphate" straddling Syria and Iraq.

In 2015, Russian security services estimated that 2,900 Russian citizens had joined the jihadist group, as well as several thousand Central Asians.

In a phone call on Thursday, Trump and Putin discussed the crisis over North Korea's nuclear programme, and the US leader took the unusual step of thanking his Russian opposite number for hailing the American economy.

The pair have lavished praise on each other in the past, with commentators describing their cosy relationship as a "bromance."

- Diplomatic ties still fraught -

But diplomatic ties between Washington and Moscow are still fraught, with both expelling some of each other's diplomats in September and the US designation last month of Russia's English-language news channel RT as a "foreign agent."

Nonetheless, "President Trump appreciated the call and told President Putin that he and the entire United States intelligence community were pleased to have helped save so many lives," the White House said.

"President Trump stressed the importance of intelligence cooperation to defeat terrorists wherever they may be."

He then called CIA Director Mike Pompeo to "congratulate him, his very talented people, and the entire intelligence community on a job well done!" the statement added.

At his annual press conference this week, Putin said allegations of Russian interference in last year's US election had been "made up by people who are opposed to Trump so as to delegitimize his work."

The two leaders met in July on the sidelines of the G20 summit in Germany, after which Trump said he "accepted" Putin's assurance that Moscow did not meddle in the vote.

"The Trump that you see on TV is very different than the real Trump," Putin told reporters at the time. "There is every reason to believe that we will be able to at least partially re-establish the level of cooperation that we need."