The US Postal Service has sent letters to most states warning that millions of ballots cast by mail for November's presidential election may not arrive in time to be counted, US media reports said.
The letters, dated July 29 and delivered to election officials in 46 states and in Washington DC, said that even if voters meet their state deadlines, the Postal Service could not guarantee delivery in time, the Washington Post said on Friday.
They warn that "certain deadlines for requesting and casting mail-in ballots are incongruous with the Postal Service's delivery standards," according to several letters posted on the USPS website following an information request by the newspaper.
"This mismatch creates a risk that ballots requested near the deadline under state law will not be returned by mail in time to be counted under your laws as we understand them."
Due to the coronavirus pandemic there are likely to be an unprecedented number of postal votes in what is set to be a very contentious election.
Facing an uphill battle to retain the White House, President Donald Trump has launched a battle against mail-in voting, which he fears would favor Democratic rival Joe Biden.
Former president Barack Obama has criticized Trump's "attempts to undermine the election," tweeting Friday that the administration was "more concerned with suppressing the vote than suppressing a virus."
"If you're in a state where you have the option to vote early, do that now," he said on Twitter.
Trump openly acknowledged this week that restricting the US Postal Service, including depriving it of resources in the run-up to the election, would impact vote-by-mail efforts.
He installed Republican fundraiser Louis DeJoy, who previously ran a freight shipping firm, as postmaster general in early May.
DeJoy has clamped down on overtime pay and hiring of mail carriers, which critics say have contributed to slowing deliveries at a time when voting by mail will be critical.
The president has repeatedly claimed that mail-in voting leads to massive fraud, a claim not supported by any evidence from states already offering voting by mail.
Despite the warning letters, the USPS said it was "well prepared and has ample capacity to deliver America's election mail," in a statement quoted by CNN.
"However, the increases in volume and the effect of when volumes were mailed in the primary elections presented a need to ensure the Postal Service's recommendations were reemphasized to elections officials."
Trump -- who himself has voted by mail -- has made clear he believes that the Democrats will reap more votes if the practice is universal.
Mail-in voting "doesn't work out well for Republicans," he said in April.