The deal between world powers and Iran has delayed Tehran's acquisition of nuclear weapons by 10 to 15 years, the head of a top defence think-tank told AFP on Tuesday.

The agreement struck in Vienna in July between Iran and the permanent five members of the UN Security Council plus Germany (P5 plus one) sees sanctions progressively lifted in return for Tehran ensuring its nuclear programme remains for civilian use.

"2015 was by and large a decent year for news on the proliferation of weapons of mass destruction," said John Chipman, the director-general and chief executive of the International Institute for Strategic Studies (IISS).

"Principally because, of course, we did see in the last year the conclusion of the P5 plus one agreement on the Iranian nuclear file."

He was speaking after presenting the IISS's annual assessment of the global balance of military power.

"There are lots of sceptics as to how good an arrangement that is but there is some consensus that at least that it has delayed for some 10 years, and possibly 15 years, the acquisition by Iran of a nuclear weapon," he added.

"That's good proliferation news for the year 2015-2016."