The White House acknowledged on Friday that "serious gaps" remain in deadlocked talks over Iran's nuclear program just days before a deadline for an agreement, Agence France Presse reported.
"We are running against the clock. Obviously, the deadline is Monday, and our folks there are working furiously to meet it... Serious gaps do remain," said spokesman Eric Schultz.
The comments came as US Secretary of State John Kerry and Iran's foreign minister Mohammed Javad Zarif opted to stay on in Vienna in an attempt to achieve a breakthrough.
Iran and the six powers - the US, China, Russia, Britain, France plus Germany (P5+1) - have been negotiating intensively since February to turn an interim accord reached with Iran a year ago into a lasting agreement by November 24.
The deal is aimed at easing fears that Tehran will develop nuclear weapons under the guise of its civilian activities - an ambition the Islamic republic hotly denies.
Schultz said negotiators would not allow the fast-approaching deadline to force them into accepting an unsatisfactory deal.
"We've been clear that we won't - the president has been clear we won't take a bad deal," he said.
"So we're going to work against the clock. We're only going to approve a deal that effectively cuts off all pathways to a nuclear weapon."