LAUSANNE (AFP) - Down-to-the-wire nuclear talks with Iran remained blocked on three major issues, a Western diplomat said Monday on the eve of a deadline to agree the outlines of a deal.

It was "yes or no" time, the diplomat said, adding that hurdles still remained over how long any deal should last, the lifting of UN sanctions, and a mechanism for ensuring both sides keep up their end of the deal and for snapping sanctions back into place.

"There can only be a deal if we can resolve these issues," the diplomat said, asking not to be identified.

"At some point you have to say 'yes or no'," he said, adding that conditions for a deal "were more favourable today than three months ago".