“I have seen so many indictments and accusations against Russians,” Mr. Kislyak said on Saturday afternoon. “I am not sure I can trust American law enforcement to be the most truthful source against Russians.” He added, “The allegations being mounted against us are simply fantasies.”

Mr. Kislyak, who has been caught up in the investigation because of meetings with Trump campaign officials during his time as ambassador, went on to cite a study, which he said he was keeping in his briefcase, that proved the “main source of computer attacks in the world is not Russia. It is the United States.”

The accusations and counteraccusations about cyber- and social media attacks were the main point of division between the Americans and the Russians, but hardly the only ones expressed at the conference.

Mr. Kislyak argued that the Trump administration’s new nuclear strategy involved the manufacture of low-yield weapons and made nuclear war more probable. The Americans argued they were just matching Russian capabilities and charged that Moscow was in continued violation of the Intermediate Nuclear Forces agreement, limiting tactical weapons.

Yet the cyberattacks on the Democratic National Committee and others, and the use of Facebook, Twitter and other social media platforms to spread propaganda, dominated the discussion and reflected how quickly the use of these techniques has become the new battleground in a very different kind of post-Cold War conflict.

The White House cybercoordinator, Rob Joyce, was particularly direct in his accusations that Russia was behind the broad attack on Ukraine last June, called “NotPetya.” He described the attack as “indiscriminate” and noted that it paralyzed operations far beyond Ukraine, the intended target, and included the Maersk shipping system. He said the United States would retaliate, but did not say how, adding “we will not telegraph these punches.”

Mr. Joyce is no newcomer to offensive cyberoperations; previously he ran the Tailored Access Operations unit of the National Security Agency, overseeing American cyberaction against other counties. “We are going to conduct cyberoperations,” he said. But “we need to do it in a responsible, balanced way.”