Any military intervention in the Caspian Sea area would be unacceptable, Mr Putin declared as he attended a five-country regional summit in Tehran.

"We should not even think of making use of force in this region," Mr Putin told his fellow leaders.

His remarks also appeared directed at Azerbaijan, amid Russian media speculation that the US might be trying to negotiate with the republic on the right to use military facilities there, something Azeri officials deny.

"We are saying that no Caspian nation should offer its territory to third powers for use of force or military aggression against any Caspian state," Mr Putin said.

A summit declaration from the five, which include Turkmenistan and Kazakhstan beside Russia, Iran and Azerbaijan, subsequently stressed that "under no circumstances will they allow (the use of their) territories by third countries to launch aggression or other military action against any of the member states".

The first Kremlin leader to visit Iran since Joseph Stalin in 1943, Mr Putin is holding talks with Mahmoud Ahmadinejad, the Iranian president, on Tehran's nuclear dispute with the west during the summit.

The importance of Mr Putin's trip to the Iranian side was clearly illustrated by the presence of Manouchehr Mottaki, Iran's foreign minister, who greeted Mr Putin as he stepped of his plane after it landed at Tehran's Mehrabad airport at around 9am local time (6.30BST).

Leaders from Azerbaijan, Kazakhstan and Turkmenistan were met by lower-level dignitaries.

Mr Putin was later greeted personally by Mr Ahmadinejad, before the start of the summit at the palatial Saad Abad complex in north Tehran, local media reported.

Mr Putin has been accompanied by a retinue of 180 staff, aides and security personnel. A fleet of armoured cars has also been shipped from Russia for his trip.

The Russian leader's arrival had earlier been in doubt after the Russian special security services disclosed on Sunday that they had uncovered a plot to assassinate him in a suicide bomb in the Iranian capital. Iranian officials angrily dismissed the alleged conspiracy as "psychological warfare".

Mr Putin's presence was widely expected to overshadow the official purpose of the conference, which is ostensibly to thrash out disagreements over the sharing of resources in the oil-rich Caspian Sea.

Iran sees the arrival of Mr Putin - who accepted Mr Ahmadinejad's invitation to visit when the pair met at an Asian co-operation summit in Shanghai in August - as a major coup in its efforts to resist western pressure over its nuclear programme, which the US and its allies suspect is aimed at building an atomic bomb.

Russia's veto on the UN security council means its support is vital if Iran is to avoid a third round of sanctions when the body meets next month.

Moscow has criticised US attempts to pressure Tehran through economic sanctions and military threats and has called for greater engagement with Iran.

Last week, Mr Putin angered the US by declaring that there was no "objective evidence" that Iran is trying to develop a nuclear bomb.

However, he has also called on Iran to show greater openness and flexibility over its uranium enrichment programme.

The nuclear issue is not on the summit agenda, but it will be discussed when Mr Putin holds one-on-one meetings with Mr Ahmadinejad and Ayatollah Ali Khamenei, Iran's supreme leader, who has the final say on all state matters.

Despite the official hype, Mr Putin's visit is being treated with scepticism. Russia is mistrusted by many Iranians, who point to a history of hostile relations going back two centuries.

Iran is angry over delays in a £500m contract with Russia to build the Bushehr nuclear power station. Russia has put completion on hold because of alleged late payments by the Iranians. Iran has denied the accusation and believes Russia is merely trying to use the issue to wring concessions over the nuclear programme.

The historic suspicions were reflected in an article today on the reformist No Rouz website, headlined The Second Turkmanchai Is On Its Way, a reference to an 1828 peace treaty in which Iran was forced to cede large swathes of territory in the Caucasus to Russia.

The article claimed Iran was being pressed to accept a "shameful" agreement giving it an 11% share of Caspian resources in a package that would allow Russian nuclear submarines to sail close to the Iranian coast.

Until 1991, Tehran and Moscow had equal shares in the Caspian Sea, but the break-up of the Soviet Union triggered moves to re-allocate resources.

Iran says the resources should be shared equally among the five Caspian states, but Russia believes it should be based on coastal size, giving Tehran a much smaller proportion and depriving it of gas and oil resources.

Opening today's conference, Mr Ahmadinejad ignored such difficulties. "The five countries are all cradles of civilisation and have existed in peace for thousands of years," he said.