Asked by a reporter if Russia’s actions amounted to a cyberwar, Mrs. Clinton said, “I’m not comfortable using the word ‘war.’”

But she said she viewed the hacking of the Democratic National Committee, which American officials have linked to Russian intelligence agencies, and warnings there could be additional breaches intended to influence the presidential campaign “a threat from an adversarial foreign power.”

In conversations lately with donors, Mrs. Clinton has urged noncomplacency. She has stressed that foreign enemies who hope Mr. Trump will succeed, in order to weaken the United States, may try to disrupt the November results, perhaps with a cyberattack.

Hours after President Obama met with Mr. Putin at the Group of 20 summit in Hangzhou, China, Mrs. Clinton delivered a harsh assessment of the Russian leader’s role in the hacking of the Democratic National Committee and potential future breaches.

“When Putin was asked about it, he could barely muster the energy to deny it, if any of you saw it,” Mrs. Clinton said, adding that the Russian president, whom she clashed with as secretary of state, “and the team around him certainly believe that there is some benefit to them to doing this.”