While the two countries are driven by different motives and have spent much of the conference deriding each other's positions, they could together end up forcing the conference to a stalemate, the activists said on Friday.
The meeting began on 2 May and ends next Friday, leaving it one final week to reach an agreement.
"I think it's a very real possibility that some states are going to try to make the clock run out," said Rebecca Johnson, co-founder of the London-based Acronym Institute for Disarmament Diplomacy.
Treaty at stake
"If this review ends in a shambles ... the danger is that you will have confidence in the treaty eroding," she said.
The conference became bogged down in wrangling for its first two and a half weeks, first over the agenda and then over the allocation of work among treaty committees. While the disputes played out behind the scenes, the activists blamed the delays on Tehran and Washington.
The United States spent the first two weeks of the conference quietly seeking to block discussions of nuclear disarmament-related commitments and decisions reached at 1995 and 2000 NPT review conferences.
"I think it's a very real possibility that some states are going to try to make the clock run out"
At the same time, Iran sought to block discussion of its nuclear enrichment programme, which it insists is intended to produce fuel for nuclear power plants, while the United States, Britain, France and Germany say it is aimed at making bombs.
Washington is providing a selective presentation of its record at the conference, said Daryl Kimball of the Washington-based Arms Control Association.
Its public stance "does not hide the fact that it has taken actions contrary to US disarmament commitments and obligations established by the NPT and the 1995 and 2000 NPT review conferences," he said in a statement.
These include its publicly stated opposition to the Comprehensive Test Ban Treaty, its pursuit of a new generation of deep burrowing nuclear bunker-busting weapons, and its resistance to agreeing to deep and verifiable nuclear weapons reductions, Kimball said.
On Iran, the conference should agree to freeze construction of new facilities capable of producing highly enriched nuclear fuel which is needed for weapons but not required to produce power, he said.