Russia and the United States have been mulling joint military operations in Syria but in an unprecedented move on Tuesday, Moscow used bases in Iran to launch air strikes against militants in Syria.
The move underscored Moscow's increasingly close ties with Tehran and 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 Bashar al-Assad's regime in September last year.
The long-range Russian Tupolev-22M3 bombers and Sukhoi-34 fighter bombers took off from Iran's Hamadan air base to strike a range of targets in Syria.
The Russian Defence Ministry said its bombers conducted strikes targeting DAESH 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 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.
Relations between Tehran and Moscow have grown warmer since Iran reached agreement last year with global powers to curb its nuclear programme in return for the lifting of UN, EU and US 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 US 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 an agreement with Russia on military cooperation in the fight against DAESH in Syria. Toner said those talks continued despite stepped up Russian-Iranian cooperation.
— (@mod_russia) August 16, 2016
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 US-led coalition against DAESH flying to Syria from Kuwait.
Russia also gave advance notice to the US-led coalition battling DAESH in Syria and Iraq, complying with the terms of a safety agreement meant to avoid an accidental clash in the skies, said US Army Colonel Christopher Garver, a Baghdad-based spokesman for the US-led coalition.
A US official, speaking on condition of anonymity, said the Russian bombers were believed to have returned to Russia.
The Syrian Observatory for Human Rights, a UK-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 regime-held zones. Assad's regime 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 regime closes the corridor linking it with the outside.