The Donald 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 is a Washington fixture with a sprawling network, and he has emerged as the central figure in the investigations into Trump advisers' connections with Russia.

In a matter of weeks, contact with 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.

Separately, a White House official confirmed that Trump's son-in-law Jared Kushner and ousted national security adviser Michael Flynn met with Kislyak at Trump Tower in December for what the official called a brief courtesy meeting.

Flynn was pushed out of the White House last month after officials said he misled Vice-President Mike Pence about whether he and the ambassador had discussed U.S. sanctions against Russia in a phone call.

USA Today has reported that J.D. Gordon and Carter Page, two members who served on the Trump campaign's national security advisory committee, also met with the Russian ambassador last July.

The Washington website Politico.com calls Kislyak "Washington's most dangerous diplomat."

Donald Trump's former national security advisor Michael Flynn, right, shakes hands with Russian President Vladimir Putin, in Moscow. Flynn resigned following reports that he misled White House officials about his contacts with Kislyak. (Associated Press)

At issue Thursday were two meetings between Sessions and Kislyak — one in July and another in September, at the height of concern over Russia's involvement in hacking of Democratic officials' emails accounts. Intelligence officials have since concluded that Moscow ordered the hacks to tilt the election toward Trump.

In his confirmation hearing, the Alabama Republican denied having contact with any Russian officials, neglecting to mention the meetings with Kislyak, which were first reported by the Washington Post.

The Russian Embassy did not respond to a request for comment.

Although the White House dismissed the revelation as part of a political witch hunt, Sessions's former colleagues took the omission seriously. At the urging of some in his own party, Sessions recused himself from the Department of Justice's investigation. Still, Democrats called for him to step down.

Russian Foreign Ministry spokeswoman Maria Zakharova: 'I'll open a military secret for you ….' (Alexander Nemenov/AFP/Getty Images)

Observers note Kislyak is a somewhat unlikely figure to cause controversy. Over the course of a long diplomatic career, he's led the life of a typical global envoy — making himself a reliable presence on the circuit of receptions, teas and forums that make up the calendar of any ambassador.

Appointed in 2008, Kislyak is regularly spotted walking around town, heading to and from meetings. Early in his tenure, he often opened the doors of the Russian Embassy, hosting dinners for foreign policy professionals, Pentagon officials, journalists and Capitol Hill staffers.

Those who have attended the events describe him as a gracious and amiable diplomat, although perhaps not as polished — nor as confrontational — as his more famous boss, Foreign Minister Sergey Lavrov.

First went to New York

Kislyak, 66, has bounced between the United States and Russia for most of his long career.

His first foreign posting was to New York where he worked at the Soviet delegation at the United Nations in the early 1980s. He spent the following years as the first secretary and then councillor at the Soviet Embassy in Washington before returning to Moscow in 1989, where he took a succession of senior jobs at the Foreign Ministry.

I'll open a military secret for you: It's the diplomats' jobs to have contacts in the country they are posted to. - Maria Zakharova , Russian foreign ministry

He did a stint as Russian ambassador to Belgium and simultaneously served as Moscow's envoy at NATO. He then returned to Moscow to serve as a deputy foreign minister, overseeing relations with the United States and arms control issues before being sent to Washington.

Kislyak's contacts have sparked questions about his role or involvement in the hacking, questions that are difficult to answer.

The U.S. and Russia, along with many other countries, have made it a practice to separate their top diplomats from espionage activities, although it is not uncommon for an intelligence agent to operate under the cover of a senior-level diplomat.

Not believed to be spies

Foreign diplomats to the United States likely expect that their activities will be monitored by U.S. authorities in the same manner that American diplomats are monitored in countries like Russia.

Russian ambassadors are most likely aware of the intelligence agents operating under diplomatic cover, but are not believed to part of the security services themselves.

Russian Foreign Ministry spokeswoman Maria Zakharova on Thursday ridiculed the claims of Kislyak's involvement in espionage as "total disinformation" and part of efforts to sway public opinion.

"I'll open a military secret for you: It's the diplomats' jobs to have contacts in the country they are posted to," she said sarcastically. "It's their obligation to meet with officials and members of the political establishment."