WASHINGTON (AFP) - Talks on a nuclear deal with Iran have made "real progress" but several issues remain, the White House said Monday, suggesting marathon negotiations in Vienna could stretch into another day.

"They have made genuine progress... but there continues to be some sticking points that remain unresolved," White House spokesman Josh Earnest told reporters in Washington.

"The president has directed his team to remain engaged and participate in conversations as long as those conversations remain useful."

Diplomats from the United States, Russia, China, Britain, France and Germany are negotiating with Tehran for a deal that would see Iran scale down its atomic activities in order to make any drive to make the atomic bomb all but impossible.

The current diplomatic effort began when Iranian President Hassan Rouhani came to power in 2013. In November that year an interim deal was agreed but two deadlines in 2014 for a lasting accord were missed.

Much of the technical work in what will be a highly complex accord is done, but the talks have stumbled on the exact timing of sanctions relief and Iran's desire to have a UN conventional arms embargo lifted.