The destruction of the last known stocks of live smallpox virus in the US and Russia has been put off for another three years.

Delegates at an annual meeting of the World Health Organization agreed to delay the demolition until 2014, indicating that more research needs to be carried out in case the disease returns, possibly as a biological weapon.

The US said further vaccines and anti-virals against smallpox need to be developed, and 27 countries backed its position, Reuters reports.

Iran led opposition to a plan to set a date for destruction in 2016, but this was only backed by seven countries.

Smallpox, an infectious disease caused by either of two virus variants, Variola major and Variola minor, was eradicated 30 years ago.

The debate to destroy the last remaining stocks began at the World Health Assembly in 1986.