The final preparatory meeting for next year's review of the Nuclear Nonproliferation Treaty has ended with deep divisions, and U.S. Ambassador Robert Wood says reaching agreement at the 2020 conference "will be an incredibly difficult task."

But he told Friday's closing session of the two-week preparatory conference that "it is a task we cannot abandon."

The NPT is the world's single most important pact on nuclear arms, credited with preventing their spread to dozens of nations since entering into force in 1970 and next year is the treaty's 50th anniversary.

Ambassador Syed Hasrin Syed Hussin, chair of the third preparatory conference, told a news conference that the meeting created "significant challenges," especially because of differences between nuclear weapon and non-nuclear weapon states on disarmament.