Iran believes that barriers to the revival of its 2015 nuclear accord with world powers are complicated but not insurmountable, a spokesman said on Tuesday, denying that negotiations had stalled.

Iran and six powers have been negotiating in Vienna since April to work out steps for Tehran and Washington to take, respectively, on nuclear activities and sanctions, for the pact to resume.

"There is no impasse in the Vienna talks," Iranian government spokesman Ali Rabiei told a news conference streamed live by a state-run website.

"Negotiations have reached a stage where a few key issues need to be decided, and these issues require the proper attention, perfectionism and time."