The United States and Russia have signed a new Strategic Arms Reduction Treaty (Start), as world leaders gear up for next week's nuclear security summit in Washington.

Al Jazeera's Divya Gopalan speaks to Gareth Evans, co-chair of the International Commission on Nuclear Non-proliferation and Disarmament, on the latest developments.

He says the US and Russia must be serious about reducing the number of nuclear weapons and momentum must be maintained to achieve the target of reducing nuclear stockpiles by 90 per cent by 2025.

Source: Al Jazeera