The Trump administration's back-to-back controversies over its Russian ties now have at least one thing in common: Ambassador Sergey Kislyak.

Moscow's top diplomat in the US has become the Kevin Bacon of the Trump White House's Russia imbroglio. A Washington fixture with a sprawling network, he has emerged as the central figure in the investigations into Trump advisers' connections with Russia.

In a matter of weeks, contact with Mr Kislyak led to the firing of a top adviser to the president and, on Thursday, prompted calls for the attorney general Jeff Sessions to resign.