US federal election officials have said the 2020 White House vote was the "most secure in American history", rejecting President Donald Trump's fraud claims.
"There is no evidence that any voting system deleted or lost votes, changed votes, or was in any way compromised," the committee announced.
They spoke out after Mr Trump claimed without proof 2.7 million votes for him were "deleted" in last week's election.
He has yet to concede to the projected winner, Democrat Joe Biden.
The result was called by US media last weekend but some counting continues.
Mr Trump has launched a flurry of legal challenges to projected results in key states and levelled unsubstantiated allegations of widespread electoral fraud.
What did the election officials say?
A Department of Homeland Security unit that worked on safeguarding US voting systems for the November 3 presidential election issued a joint statement on Thursday.
The committee of the Cybersecurity and Infrastructure Security Agency (CISA) said: "While we know there are many unfounded claims and opportunities for misinformation about the process of our elections, we can assure you we have the utmost confidence in the security and integrity of our elections, and you should too.
"When you have questions, turn to elections officials as trusted voices as they administer elections."
The head of CISA, Christopher Krebs, has said he expects to be fired by the Trump administration, according to Reuters news agency.
It reported that Mr Krebs had incurred the White House's displeasure over a CISA website called Rumor Control, which debunks election misinformation.
On Thursday, Mr Krebs shared a post by an election law expert that said: "Please don't retweet wild and baseless claims about voting machines, even if they're made by the president."
CISA assistant director Bryan Ware stepped down on Thursday. The White House had asked for his resignation earlier this week, Reuters reports.
In a separate development, former President Barack Obama - a Democrat - said senior Republicans were undermining democracy by going along with President Trump's claims of fraud.
"It's one more step in delegitimising not just the incoming Biden administration, but democracy generally, and that's a dangerous path," he told CBS News ahead of the release of his new memoir, A Promised Land.
How are Republicans reacting?
A small but growing number of Republicans are backing calls for the president-elect to be given daily intelligence briefings.
Lindsey Graham, a key Trump ally, was among those saying Mr Biden should get the secret presidential memo, as is usual with incoming presidents.
Chuck Grassley, John Cornyn and John Thune agreed, although House Minority Leader Kevin McCarthy said Mr Biden was "not president right now" and should wait.
Between 10 and 20 Republicans in Congress have now either congratulated Mr Biden or accepted there must be moves towards a transition. But most have yet to acknowledge the president-elect's win.
The president tweeted on Thursday that voting software used in 28 states had deleted millions of votes for him, but presented no evidence for the stunning claim.
The Democrats' House Speaker Nancy Pelosi said the "absurd circus" meant the coronavirus pandemic was being neglected, and she and other top Democrats urged Republicans to "accept reality".
Democratic Senator Chris Coons told CNN some Republicans had been asking him to congratulate Mr Biden on their behalf because they did not feel able to do so publicly.
Why are Republicans holding back?
The Democratic Senate minority leader Chuck Schumer said they were "deliberately casting doubt on our elections for no other reason but fear of Donald Trump".
Capitol Hill reporters say Republican lawmakers are anxious not to alienate the Trump base, given that the president just won more votes than any incumbent ever, even though he is projected to lose.
Party bigwigs are also said to be hoping that Mr Trump will help campaign for two Senate run-off elections in January in Georgia that will decide whether Republicans retain control of the upper chamber.
Mr Biden is 5.2 million votes ahead of Mr Trump - about 3.4 per cent - and has enough electoral votes needed in the state-by-state Electoral College to take the presidency.
President Trump has kept a low key public profile since the election. Reports suggest he has told friends he wants to start a digital media company to take on the conservative network Fox News, whose full support he now feels to be lacking.
According to CBS News, Mr Trump is also openly discussing a possible 2024 campaign to retake the presidency.
What has Biden been up to?
On Thursday he spoke with Pope Francis, who offered Mr Biden his "blessings and congratulations". Mr Biden will be only the second Roman Catholic president of the US.
He also spoke with congressional Democratic leadership about the need for a coronavirus stimulus package as the daily US caseload from the disease soared to a new record of more than 150,000.
He spent the day huddled with his transition team in Wilmington, Delaware, where he had been planning cabinet appointments.
On Wednesday he picked veteran Democratic operative Ron Klain to be his White House chief of staff.