WASHINGTON (NYTIMES) - Sergey Kislyak, the longtime Russian ambassador to the United States, hosted a dazzling dinner in his three-storey, beaux-arts mansion four blocks north of the White House to toast Michael McFaul just weeks before he took up his post as the US envoy to Russia.

It was, McFaul recalled, an "over-the-top, extraordinary dinner," including five courses of Russian fusion cuisine for 50 seated guests who shared one main characteristic: They were government officials intimately involved in formulating Russia policy for the Obama administration, including senior figures from the Defence and State departments.

"I admired the fact that he was trying to reach deep into our government to cultivate relations with all kinds of people," McFaul said of the dinner in late 2011. "I was impressed by the way he went about that kind of socialising, the way he went about entertaining, but always with a political objective."

Kislyak's networking success has landed him at the centre of a sprawling controversy and made him the most prominent, if politically radioactive, ambassador in Washington.

Two advisers to President Donald Trump have run into trouble for not being more candid about contacts with Kislyak: Michael Flynn, who was forced to resign as national security adviser, and now Attorney General Jeff Sessions, who admitted two previously undisclosed conversations.

Kislyak also met during the transition with Trump's son-in-law and adviser, Jared Kushner.

A career diplomat raised in the Soviet era, Kislyak, 66, may seem an unlikely protagonist in such a drama. He has interacted with US officials for decades and been a fixture on the Washington scene for the past nine years, jowly and cordial with an easy smile and fluent if accented English, yet a pugnacity in advocating Russia's assertive policies.

Invited to think tanks to discuss arms control, he would invariably offer an unapologetic defence of Russia's intervention in Ukraine and assail Americans for what he portrayed as their hypocrisy - then afterward approach a debating partner to suggest dinner.

"Not all of us, myself included, initially appreciated his very tough, in-your-face style," said Dimitri Simes, president of the Centre for the National Interest and an advocate of closer Russian-American relations, who hosted a dinner at his home for Kislyak after his arrival in Washington and regularly invited him to events at his center.

"But we gradually came to develop a grudging respect for him as someone who was really representing the positions of his country."

Simes introduced Kislyak to Trump in a receiving line last April at a foreign policy speech hosted by his centre at the Mayflower Hotel in Washington. Kislyak was one of four ambassadors who sat in the front row for Trump's speech at the invitation of the center.

Simes noted that Sessions, then a senator from Alabama, was there, but he did not notice whether he and the ambassador spoke at that time.

The Russian Embassy did not respond to an email on Thursday (March 2), but Kislyak defended engagements with US officials last November, when he was asked during a speech at Stanford University about allegations of Russian meddling in the elections.

Kislyak echoed his government's line that it was not involved in hacking. He said it was natural for diplomats to attend events such as political conventions and foreign policy speeches by candidates.

"It is normal diplomatic work that we have been doing: It is our job to understand, to know people, both on the side of the Republicans and Democrats," he said. "I personally have been working in the United States for so long that I know almost everybody."

Even some critics of Russian policy said it was hardly surprising that Kislyak would meet people around Trump.

"That was part of his job," said Steven Pifer, a former ambassador to Ukraine who is now at the Brookings Institution. "I don't see anything nefarious in that per se, and I don't think it was out of the box for Senator Sessions to talk with Kislyak."

An expert on arms control negotiations with a degree from the Moscow Engineering Physics Institute, Kislyak first served in the Washington embassy from 1985-89 during the late Soviet period. He became the first Russian representative to Nato and was ambassador to Belgium from 1998 to 2003. He returned to Moscow, where he spent five years as a deputy foreign minister.

He was appointed ambassador to Washington in 2008.

The questions about contacts between Trump's circle and Russian officials have revealed what both sides presumably knew, that US intelligence agencies closely track Kislyak's movements and tap his phone calls. Russian officials on Thursday expressed anger that their ambassador's actions were being questioned and that some news reports suggested he might be an intelligence operative.

Maria Zakharova, the spokesman for the Russian Foreign Ministry, delivered an extended diatribe during her weekly briefing against what she called the low professional standards of the US news media.

"I will reveal a military secret to you: Diplomats work, and their work consists of carrying out contacts in the country where they are present," she said. "This is on record everywhere. If they do not carry out these contacts, do not participate in negotiations, then they are not diplomats."

Kislyak has told associates that he will leave Washington soon, likely to be replaced by a hardline general. His name recently surfaced at the United Nations as a candidate for a new post responsible for counterterrorism, diplomats there said.

Vitaly Churkin, the Russian ambassador to the United Nations, died last month and that post remains vacant.

For Kislyak, Washington is no longer the place it once was. It has become lonely, and he has told associates that he is surprised how people who once sought his company were now trying to stay away.