Officials hope for breakthrough after almost 18 months of negotiations.
Says deal to transform the relationship between U.S. and Iran could be within reach.
John Kerry due for talks in Geneva with his Iranian counterpart.
U.S. and Iran agree to a seven-month extension to talks.
As Nov. 24 deadline approaches, officials say another interim pact likely.
President Hassan Rouhani said he believed an agreement could be achieved by a Nov. 24 deadline.
Talks between David Cameron and Hassan Rouhani were Britain's first with Iran leadership since 1979.
Iran and six world powers will hold their first negotiating round since failing to meet July target date.
Lead U.S. negotiator declined to give a final deadline for final nuclear deal.
As deadline nears, U.S. Secretary of State John Kerry said "significant differences" remain.
