The five Caspian Sea state leaders have agreed a statement supporting Iran's right to nuclear energy.
At a summit in Tehran, they backed the right of Non-Proliferation Treaty members to research, produce and use nuclear energy for peaceful ends, without discrimination, within the framework of the treaty.
The US claims Tehran only wants nuclear energy as cover for an atomic weapons programme, a charge which Iran strenuously denies.
Amid continued speculation over the possibility of a US military strike against Iran, the Caspian leaders also agreed never to allow their territory to be used for an attack on a fellow littoral state.
Russian President Vladimir Putin told the summit the Caspian Sea should unite rather than divide them.
The group comprises the heads of state of Azerbaijan, Iran, Kazakhstan, Russia, and Turkmenistan.
Mr Putin arrived in Tehran early this morning, after doubts that he would travel when rumours of an assassination plot against him were widely disseminated.
He was welcomed by Foreign Minister Manouchehr Mottaki to the Iranian capital.
As well as attending the summit, the Russian leader will meet Iranian president Mahmoud Ahmadinejad and supreme leader Ayatollah Ali Khamenei.
A major issue will be the much delayed project for Russia to build Iran's first nuclear power station in the southern city of Bushehr.
The project has not yet been completed and there has been a series of missed deadlines.
