The Department of Homeland Security has not detected attempts by foreign adversaries, including Russia and China, intended to penetrate American election infrastructure a month ahead of the 2018 midterm elections, though the possibility still exists that they and other actors may still make those kinds of attempts, the head of the department said Tuesday.

"We currently have no indication that a foreign adversary intends to disrupt our election infrastructure," said DHS Secretary Kirstjen Nielsen at a cybersecurity summit hosted by the Washington Post. "This is a point in time. We know they have the capability and we know they have the will," she added, "so we're constantly on alert to watch."

Nielsen also described China's efforts to date as belonging to "traditional," "nefarious" and "nebulous" forms of foreign influence campaigns, a characterization that marked a departure from the president's starker allegation last week.

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As he presided over a security council session at the United Nations General Assembly, President Trump explicitly accused China of "attempting to interfere in our upcoming 2018 election" against his administration and as apparent retaliation for the United States' escalating trade tensions with China. "They do not want me or us to win because I am the first president to ever challenge China on trade," he said.

Mr. Trump later tweeted, as partial evidence, images of a paid newspaper ad in the Des Moines Register, and said in a press conference that more evidence was forthcoming, without offering details. "We have evidence," Trump said at the time. "It will come out. I can't tell you now."

Nielsen did not offer new evidence Tuesday, though she suggested China had continued known and longstanding efforts to boost its own favorability. "It's part of a more holistic approach to influence the American public in favor of China," she said, adding that Russia's activities were, comparatively, "pretty noisy."

"Russia is more, at the moment, focused on sowing discord on all sides and through that chaos hoping to promote their own policies," she said. "China is playing, perhaps, a longer game."

The U.S. intelligence community concluded with high confidence in an assessment issued in 2017 that Russia, at President Vladimir Putin's instruction, conducted a targeted influence campaign intended to bolster Mr. Trump's electability and damage Secretary Clinton's. That campaign involved hacking the email systems of the Democratic Party and Clinton campaign officials, weaponizing the release of hacked material, and launching a coordinated social media influence campaign that reached hundreds of thousands of American voters. The assessment did not address whether Russia's activities affected the outcome of the election.

Although law enforcement and intelligence officials have said Russia's activity is not occurring at the same scale and with the same intensity ahead of the midterm elections, some interference efforts, including on social media, are ongoing. Nielsen declined to say what types of campaigns or officials had been targeted but noted spear phishing attacks had become "more and more sophisticated."

"We haven't seen any major compromises of yet," she said.

Overall, Nielsen insisted the department had made "tremendous strides" in bolstering election security considerations, including by disseminating hygiene scans and vulnerability assessments to all 50 states, speeding up information-sharing, and introducing network intrusion sensors that will cover "about 90 percent" of areas where people will vote in November.

She also said the department was setting up a "situational awareness room" in Washington, D.C., to allow for coordinated, rapid responses on election day.

"We really and truly are throwing everything we have at it," she said.