Postmaster General Louis DeJoy on Friday denied claims he was working to undermine mail delivery, after comments by President Donald Trump raised fears the US Postal Service would be sabotaged to aid his chances in the November elections.

Americans are expected to vote by mail in massive numbers due to the coronavirus pandemic but Trump, who is trailing Democratic challenger Joe Biden in the polls, said he was opposed to more funding for the cash-strapped agency, acknowledging the money would be used to help process ballots.

Trump also has repeatedly and baselessly linked mail-in voting to election fraud. 

That sparked a political firestorm and lawmakers called on DeJoy to testify before Congress, where he denied allegations that recent changes at USPS were designed to slow down election mail, calling them "outrageous."

"I want to assure this committee and the American public that the Postal Service is fully capable and committed to delivering the nation's election mail securely and on time," he told the Republican-controlled Senate Homeland Security Committee on Friday.

DeJoy, a Republican fundraiser who Trump appointed to head the USPS in May, has presided over the removal of mail collection boxes and processing equipment and cut overtime pay for mail carriers, which a union leader told AFP has slowed down delivery times nationwide.

- Plenty of capacity -

He also reshuffled senior management, and the USPS warned most states that it could not guarantee on-time delivery of mail-in ballots. 

Biden has linked the moves to the president, saying last week it was "Pure Trump. He doesn't want an election."

DeJoy told senators the plan to remove mailboxes predated his time at the agency, and said the USPS had plenty of capacity even with the sorting machines offline. 

He denied mail was being slowed down, saying changes he had made would save up to $1 billion and improve delivery times.

Targeted by protesters and coming under intense pressure by Congress, DeJoy earlier this week said some reforms would be put on hold until after the November election. 

But Democrats have said his pledge did not go far enough, and House Speaker Nancy Pelosi called the representatives back into session to address the issues. DeJoy will testify before a House committee on Monday.

A North Carolina logistics executive, DeJoy has donated $1.2 million to Trump's campaign and almost $1.3 million to the Republican party since 2016, according to The New York Times.

The US is home to the world's worst coronavirus outbreak and Americans are predicted to rely on vote-by-mail in record numbers, with an estimated three quarters of the population able to do so this fall.