Former senior advisers to U.S. President Barack Obama have expressed concern at the emerging deal between world powers and Iran over its nuclear programme, BBC News reports.

In an open letter, the security and foreign policy experts say it "falls short of meeting the administration's own standard of a 'good' agreement".

They call for fewer concessions on international nuclear inspections and on research and development activities.

The publication of the letter comes as the deadline for a final accord nears.

U.S. Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif are to fly to the Austrian capital Vienna on Friday to join the negotiations, which may need to continue beyond 30 June.

The so-called P5+1 - the U.S., UK, France, China and Russia plus Germany - wants to limit Iran's sensitive nuclear activities to ensure that it could not build a nuclear weapon quickly.

Iran, which wants sanctions lifted in exchange, insists its nuclear work is peaceful.