Donald Trump’s national security adviser has been in “very frequent” contact with Russia’s ambassador to the US, and the pair spoke on the day Barack Obama imposed sanctions on Russia for its part in election hacking, according to a senior US official.

Mr Trump’s team initially denied that Michael Flynn and Russia’s ambassador Sergey Kislyak spoke on 29 December, but later admitted that one call had been made on that day and another call made the day before.

However, Reuters reports that three different sources familiar with the situation said five calls were made between the pair on the day the sanctions were imposed.

The cosy relationship has raised questions about whether repeated contact with the President-elect’s team has helped shape Moscow’s response to the imposition of sanctions.

Trump's message to Putin

Following the enforcement of the sanctions, in which Washington expelled 35 Russian diplomats, Vladimir Putin did not retaliate, and instead said he would not deport US diplomats in response.

Mr Trump described Mr Putin’s actions as “very smart”, putting himself at odds with his own country’s government, its intelligence service and with his own party.

The President-elect’s stance has prompted outrage in Washington, including among Republicans, after US security officials advised that Russia had been behind a hacking operation aimed at influencing the US election to benefit Mr Trump.

Mr Trump has already indicated he may lift the sanctions once he takes office.

“If Russia is really helping us, why would anybody have sanctions?” he asked in an interview with the Wall Street Journal.

During an earlier press conference, he emphasised his rapport with the Russian leader.

“If Putin likes Donald Trump, I consider that an asset, not a liability, because we have a horrible relationship with Russia,” Mr Trump said.

Mr Flynn’s regular contact with Mr Kislyak suggests Mr Trump’s desire to build closer ties with Russia is well underway despite mounting concerns over the nature of the relationship.

The shifting accounts of Mr Flynn’s contact with Mr Kislyak, have also done little to alleviate the worries of many in Washington.

According to the Associated Press, one of Mr Trump’s officials confirmed Mr Flynn and Mr Kislyak had spoken by phone on 29 December, following a text message from the ambassador the day before. During the call, the Russian ambassador invited US officials to a conference on Syria later this month in Kazakhstan.

The official also confirmed a phone call between the men earlier in December.

A separate US official who spoke to the Associated Press on Friday described the contact between the two men as “very frequent”.

Boris Johnson 'not worried' about Trump and Putin's relationship

The revelations of close ties with Russia come immediately after the CIA warned that Russia may hold more than one recording of compromising footage of Mr Trump, containing video and audio of a sexual nature, which could be used to blackmail him.

The allegations follow the publication of documents compiled by a former British spy who the CIA described as a “credible” source.

Ex-MI6 officer Christopher Steele, 52, reportedly prepared the documents for Mr Trump’s Democratic and Republican opponents during the US presidential primary election.

Trump and Putin passionately kiss in street mural Show all 14 1 /14 Trump and Putin passionately kiss in street mural Trump and Putin passionately kiss in street mural A lesbian couple kisses in front of mural depicting Russian President Vladimir Putin and Republican presidential candidate Donald Trump, on the walls of a barbecue bar 'Keule Ruke' on May 19, 2016 in Vilnius, Lithuania. Barcroft Media/Getty Trump and Putin passionately kiss in street mural A lesbian couple kisses in front of mural depicting Russian President Vladimir Putin and Republican presidential candidate Donald Trump, on the walls of a barbecue bar 'Keule Ruke' on May 19, 2016 in Vilnius, Lithuania. Barcroft Media/Getty Trump and Putin passionately kiss in street mural VILNIUS, LITHUANIA - NOVEMBER 23: A woman walks past a mural showing U.S. President-elect Donald Trump (R) blowing marijuana smoke into the mouth of Russian President Vladimir Putin on the wall of a bar-b-que restaurant on November 23, 2016 in Vilnius, Lithuania. Many people in the three Baltic nations of Lithuania, Latvia and Estonia are concerned that Russia, because Trump has expressed both admiration for Putin and doubt over defending NATO member states, will be emboldened to intervene militarily in the Baltics. Sean Gallup/Getty Trump and Putin passionately kiss in street mural A woman walks past a mural on a restaurant wall depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural AP Trump and Putin passionately kiss in street mural A child walks past a graffiti depicting Russian President Vladimir Putin, left, and Republican presidential candidate Donald Trump, on the walls of a bar in the old town in Vilnius, Lithuania, Saturday, May 14, 2016. (AP Photo/Mindaugas Kulbis) AP Trump and Putin passionately kiss in street mural People walk past a mural on a restaurant wall depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural A man photographs a mural on a restaurant wall depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural AP Trump and Putin passionately kiss in street mural A young woman walks past a mural showing U.S. President-elect Donald Trump (R) blowing marijuana smoke into the mouth of Russian President Vladimir Putin with the slogan "make everything great again," in reference to Trump's campaign slogan of "Make America Great Again," on the wall of a bar-b-que restaurant on November 23, 2016 in Vilnius, Lithuania. Many people in the three Baltic nations of Lithuania, Latvia and Estonia are concerned that Russia, because Trump has expressed both admiration for Putin and doubt over defending NATO member states, will be emboldened to intervene militarily in the Baltics. Sean Gallup/Getty Trump and Putin passionately kiss in street mural A morning commuter stops to look at a mural on a restaurant wall depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural Restaurant owner Dominykas Ceckauskas pose next to a mural on the wall of his establishment depicting US Presidential hopeful Donald Trump and Russian President Vladimir Putin greeting each other with a kiss in the Lithuanian capital Vilnius on May 13, 2016. Kestutis Girnius, associate professor of the Institute of International Relations and Political Science in Vilnius university, told AFP -This graffiti expresses the fear of some Lithuanians that Donald Trump is likely to kowtow to Vladimir Putin and be indifferent to Lithuanias security concerns. Trump has notoriously stated that Putin is a strong leader, and that NATO is obsolete and expensive. / AFP / Petras Malukas (Photo credit should read PETRAS MALUKAS/AFP/Getty Images) Petras Malukas/AFP/Getty Images Trump and Putin passionately kiss in street mural A passerby photographs a mural showing U.S. President-elect Donald Trump (R) blowing marijuana smoke into the mouth of Russian President Vladimir Putin on the wall of a bar-b-que restaurant on November 23, 2016 in Vilnius, Lithuania. Many people in the three Baltic nations of Lithuania, Latvia and Estonia are concerned that Russia, because Trump has expressed both admiration for Putin and doubt over defending NATO member states, will be emboldened to intervene militarily in the Baltics. Sean Gallup/Getty Trump and Putin passionately kiss in street mural Getty

The claims were deemed grave enough for both President Obama and the President-elect to receive security briefings on the document’s content.