Russian leader Vladimir Putin met his Iranian counterpart Tuesday and implicitly warned the U.S. not to use a former Soviet republic to stage an attack on Iran. He also said nations shouldn't pursue oil pipeline projects in the area if they weren't backed by regional powers.

At a summit of the five nations that border the inland Caspian Sea, Putin said none of the nations' territory should be used by any outside countries for use of military force against any nation in the region. It was a clear reference to long-standing rumors that the U.S. was planning to use Azerbaijan, a former Soviet republic, as a staging ground for any possible military action against Iran.

"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," Putin said.

Iranian President Mahmoud Ahmadinejad also underlined the need to keep outsiders away from the Caspian.

"All Caspian nations agree on the main issue — that all aspects related to this sea must be settled exclusively by littoral nations," he said. "The Caspian Sea is an inland sea and it only belongs to the Caspian states, therefore only they are entitled to have their ships and military forces here."

Putin, whose trip to Tehran is the first by a Kremlin leader since World War II, warned that energy pipeline projects crossing the Caspian could only be implemented if all five nations that border the Caspian support them.

Putin did not name any specific country, but his statement underlined Moscow's strong opposition to U.S.-backed efforts to build pipelines to deliver hydrocarbons to the West bypassing Russia.

"Projects that may inflict serious environmental damage to the region cannot be implemented without prior discussion by all five Caspian nations," he said.

Other nations bordering the Caspian Sea and in attendance at the summit are: Kazakhstan, Turkmenistan and Azerbaijan.

The legal status of the Caspian — believed to contain the world's third-largest energy reserves — has been in limbo since the 1991 Soviet collapse, leading to tension and conflicting claims to seabed oil deposits.

Iran, which shared the Caspian's resources equally with the Soviet Union, insists that each coastal nation receive an equal portion of the seabed. Russia, Azerbaijan and Kazakhstan want the division based on the length of each nation's shoreline, which would give Iran a smaller share.

Putin's visit took place despite warnings of a possible assassination plot and amid hopes that a round of personal diplomacy could help offer a solution to an international standoff on Iran's nuclear program.

Putin's trip was thrown into doubt when the Kremlin said Sunday that he had been informed by Russian intelligence services that suicide attackers might try to kill him in Tehran, but he shrugged off the warning.

Iranian Foreign Ministry spokesman Mohammad Ali Hosseini dismissed reports about the purported assassination plot as disinformation spread by adversaries hoping to spoil good relations between Russia and Iran.

Putin has warned the U.S. and other nations against trying to coerce Iran into reining in its nuclear program and insists peaceful dialogue is the only way to deal with Tehran's defiance of a U.N. Security Council demand that it suspend uranium enrichment.

"Threatening someone, in this case the Iranian leadership and Iranian people, will lead nowhere," Putin said Monday during his trip to Germany. "They are not afraid, believe me."

Iran's rejection of the council's demand and its previous clandestine atomic work has fed suspicions in the U.S. and other countries that Tehran is working to enrich uranium to a purity usable in nuclear weapons. Iran insists it is only wants lesser-enriched uranium to fuel nuclear reactors that would generate electricity.

Putin's visit to Tehran is being closely watched for any possible shifts in Russia's carefully hedged stance in the nuclear standoff.

The Russian president underlined his disagreements with Washington last week, saying he saw no "objective data" to prove Western claims that Iran is trying to construct nuclear weapons.

Putin emphasized Monday that he would negotiate in Tehran on behalf of the five permanent U.N. Security Council members — United States, Russia, China, Britain and France — and Germany, a group that has led efforts to resolve the stalemate with Tehran.

In Washington, State Department spokesman Tom Casey said the U.S. government expected Putin to "convey the concerns shared by all of us about the failure of Iran to comply with the international community's requirements concerning its nuclear program."

Putin's schedule also called for meetings with Ahmadinejad and the Iran's Supreme Leader Ali Khamenei.

While the Kremlin has shielded Tehran from a U.S. push for a third round of U.N. sanctions, Iran has voiced annoyance about Moscow's foot-dragging in building a nuclear power plant in the southern port of Bushehr under a $1 billion contract.

Russia warned early this year that the plant would not be launched this fall as planned because Iran was slow in making payments. Iranian officials have angrily denied any payment arrears and accused the Kremlin of caving in to Western pressure.

Moscow also has ignored Iranian demands to ship fuel for the plant, saying it would be delivered only six months before the Bushehr plant goes on line. The launch date has been delayed indefinitely amid the payment dispute.

Any sign by Putin that Russia could quickly complete the power plant would embolden Iran and further cloud Russia's relations with the West. But analysts said Putin's trip would be important for Iran even if it yielded no agreements.