Federal authorities are monitoring voting in the US and any threats to the election across the country from an operations centre outside Washington DC, run by the cyber-security unit of the Department of Homeland Security.
Officials there said there were no major problems detected early on Tuesday but urged the public to be wary and patient.
US Cybersecurity and Infrastructure Security Agency director Christopher Krebs said from the centre that there was “some early indication of system disruption”, but did not elaborate.
He added that he has “confidence that the vote is secure, the count is secure and the results will be secure”.
Mr Krebs said officials have seen attempts by foreign actors “to interfere in the 2020 election”, but officials “have addressed those threats quickly” and “comprehensively”.
He added that election day “in some sense is half-time” and there “may be other events or activities or efforts to interfere and undermine confidence in the election”.
He urged all Americans “to treat all sensational and unverified claims with scepticism and remember technology sometimes fails”.
It came as Donald Trump threatened legal action to block the counting of ballots received after election day. For example, if Pennsylvania ballot counting takes several days, as is allowed, the president claimed without evidence that “cheating can happen like you have never seen”.
In fact, there are roughly 20 states that allow mail-in ballots received after election day to be counted — up to nine days and longer in some states. Litigation has centred on just a few where states have made changes in large part due to the coronavirus.
A panel of federal appeals court judges has rejected an 11th-hour Republican effort to bar drive-through voting in Houston, Texas.
The 5th US Circuit Court of Appeals denied the request in a one-sentence ruling issued late on Monday.
The request stemmed from a lawsuit brought by conservative Texas activists, who have railed against expanded voting access in Harris County, where a record 1.4 million early votes have already been cast.
The county is the nation’s third-most populous and a crucial battleground in Texas, where Mr Trump and Republicans are bracing for the closest election in America’s largest red state in decades.
Harris County Democratic Party spokeswoman Angelica Luna-Kaufman said the appeals court decision was expected following a ruling earlier on Monday by a federal judge who rejected a Republican effort to invalidate nearly 127,000 votes in Houston because the ballots were cast at drive-through polling centres established during the pandemic.
“We are not surprised that this happened… we believe the law is on the side of the voters,” Ms Luna Kaufman said. “Hopefully it’s all over now.”
Mr Trump won Texas by nine points in 2016 but polls have shown Democrat Joe Biden still within reach in America’s biggest red state. Democrats also need to flip only nine seats to reclaim a majority in the Texas House for the first time in 20 years, and have targeted several races in Harris County.