Iran's defence minister says it has successfully tested a new missile motor using solid-fuel technology with a range of 2,000km (1,250 miles).
Iran has already increased the range of the Shahab-3
Such an engine would enhance the capability of medium-range Shahab-3, which already has the ability to hit Israeli and US bases in the region.
The new engine would give the missile a greater range and increase its shelf life, Ali Shamkhani said.
Iran has taken huge steps in ballistic missile technology, experts say.
This - coupled with the country's nuclear activities - has caused alarm among the international community.
"Using solid fuel would be more durable and increase the range of the missile," Mr Shamkhani was quoted as saying by the Iranian official news agency.
Military experts are quoted as saying that the more complex solid-fuel technology would be necessary if Iran wanted to develop a two-stage rocket that could outrange the Shahab-3.
Iran says it is not seeking to develop longer-range missiles and denies allegations that it wants to make nuclear weapons.