By Andrew Osborn

MOSCOW (Reuters) - Russia used Iran on Tuesday for the first time as a base from which to launch air strikes against Syrian militants, widening its air campaign in Syria and deepening its involvement in the Middle East.

In a move underscoring Moscow's increasingly close ties with Tehran, long-range Russian Tupolev-22M3 bombers and Sukhoi-34 fighter bombers used Iran's Hamadan air base to strike a range of targets in Syria.

It was the first time Russia has used the territory of another nation, apart from Syria itself, to launch such strikes since the Kremlin launched a bombing campaign to support Syrian President Bashar al-Assad in September last year.

The Iranian deployment will boost Russia's image as a central player in the Middle East and allow the Russian air force to cut flight times and increase bombing payloads.

The head of Iran's National Security Council was quoted by state news agency IRNA as saying Tehran and Moscow were now sharing facilities.

Both countries back Assad. Russia, after a delay, has supplied Iran with its S-300 missile air defense system, evidence of a growing partnership that has helped turn the tide in Syria's civil war and is testing U.S. influence in the Middle East.

Relations between Tehran and Moscow have grown warmer since Iran reached agreement last year with global powers to curb its nuclear program in return for the lifting of U.N., EU and U.S. financial sanctions.

President Vladimir Putin visited in November and the two countries regularly discuss military planning for Syria. Iran has provided ground forces that work with local allies while Russia provides air power.

The United States said it was still assessing the extent of Russian-Iranian cooperation but described the new development as "unfortunate".

State Department spokesman Mark Toner said the United States was looking into whether the move violated UN Security Council resolution 2231, which prohibits the supply, sale and transfer of combat aircraft to Iran.

"It's unfortunate but not surprising," Toner told reporters. "It speaks to a continuation of a pattern we've seen of Russia continuing to carry out air strikes, now with Iran's direct assistance, ... that predominantly target moderate Syrian opposition forces."

He said U.S. Secretary of State John Kerry spoke by phone on Tuesday to his Russian counterpart, Sergei Lavrov, who raised the issue.

Kerry is trying to reach agreement with Russia on military cooperation in the fight against Islamic State in Syria. Toner said those talks continued despite stepped up Russian-Iranian cooperation.

TARGET: ALEPPO

Iraqi Prime Minister Haider al-Abadi said on Tuesday Iraq, which lies between Iran and Syria, had granted Russia permission to use its air space, on the condition the planes use corridors along Iraq’s borders and refrain from flying over Iraqi cities.

Abadi told a press conference the same permission has been given to air forces of a separate U.S.-led coalition against Islamic State flying to Syria from Kuwait.

Russia also gave advance notice to the U.S.-led coalition battling Islamic State in Syria and Iraq, complying with the terms of a safety agreement meant to avoid an accidental clash in the skies, said U.S. Army Colonel Christopher Garver, a Baghdad-based spokesman for the U.S-led coalition.

A U.S. official, speaking on condition of anonymity, said the Russian bombers were believed to have returned to Russia.

The Russian Defence Ministry said its bombers had taken off on Tuesday from the Hamadan air base in north-west Iran. It said the strikes targeted Islamic State as well as militants previously known as the Nusra Front in the Aleppo, Idlib and Deir al Zour provinces.

It said its Iranian-based bombers had been escorted by fighter jets based at Russia's Hmeymim air base in Syria's Latakia Province.

"As a result of the strikes five large arms depots were destroyed ... a militant training camp ... three command and control points ... and a significant number of militants," the ministry said in a statement.

The destroyed facilities had been used to support militants in the Aleppo area, it said, where battle has intensified in recent weeks for control of the divided city, which had some 2 million people before the war.

The Syrian Observatory for Human Rights, a U.K.-based war monitor, said heavy air strikes on Tuesday hit many targets in and around Aleppo and elsewhere in Syria, killing dozens.

The Russian Defence Ministry says it strives to avoid civilian casualties.

Aleppo, Syria's largest city before the war, is divided into rebel and government-held zones. The government aims to capture full control, which would be its biggest victory of the five-year conflict.

Hundreds of thousands of civilians are believed to be trapped in rebel areas, facing potential siege if the government closes the corridor linking it with the outside.

Russian media reported that Russia had also requested and received permission to use Iran and Iraq as a route to fire cruise missiles from its Caspian Sea fleet into Syria, as it has done in the past. Russia has built up its naval presence in the eastern Mediterranean and the Caspian as part of what it says are planned military exercises.

Earlier on Tuesday, Russia's state-backed Rossiya 24 channel broadcast uncaptioned images of at least three Russian Tupolev-22M3 bombers and a Russian military transport plane inside Iran. It said the Iranian deployment would allow the Russian air force to cut flight times by 60 percent.

The Tupolev-22M3 bombers, which had conducted strikes on Syria from bases in southern Russia, were too large to be accommodated at Russia's own air base inside Syria, Russian media reported.

(Additional reporting by Polina Devitt, Bozorgmehr Sharafedin, Angus McDowall, Thomas Perry, Maher Chmaytelli, Phil Stewart and Lesley Wroughton; editing by Peter Graff, Cynthia Osterman and David Gregorio)