He's the first Russian leader to visit Tehran since the second world war, and President Vladimir Putin's visit comes at a crucial time. Iran's standoff with the US and other world powers over its nuclear programme shows no sign of abating, and Russia has positioned itself as mediator. Just a few hours ago, Putin fired a warning shot at the US by making it clear that any use of military force in the Caspian region was unacceptable. Ayman Moheldine has this report. Amid much fan fare, pomp and even reports of an assassination plot the sitting president of Russia, Putin arrived in Iran. It is the first such visit by a sitting president since 1943 despite participating in a five-nation Caspian Sea Summit, it's the relations between the two that will be closely watched. Now, Moscow, which has had traditionally warm ties with Iran, will attempt to iron out possible differences over Tehran's nuclear energy program which it's helping to develop. And this is where the Russians are doing exactly that the Bushehr Power Plant. It's 90% complete but is still far from operational...Russian contractors say Iran hasn't made timely payments a claim it denies. Tehran has said Moscow was delaying construction because of intense western pressure. That has made some Iranians lose trust. It's not just bilateral issues that will dominate the headlines. Western powers will be watching the Putin's visit very closely for any signs there's a shift in Moscow's policies over Iran nuclear program - a matter Iranian President Mahmoud Ahmadinejad says is closed. While the US, UK and France have sought tougher sanctions against Iran, Russia has traditionally shied away from criticizing its nuclear energy programme. President Putin has favored diplomacy over sanctions...but if the bilateral relations between the two countries cool down, western powers may be empowered to seek the a third round of sanctions with Russian support.