Russia said Tuesday its warplanes flew out of an Iranian airbase for the first time to bomb jihadist groups in Syria, as fighting raged for control of the ravaged city of Aleppo.
The United States said the Russian move made the Syrian crisis even more difficult, but it credited Moscow with having given it a brief advance warning.
The defence ministry in Moscow said long-range warplanes took off from Hamedan base in western Iran and "conducted a group air strike against targets of the Islamic State and Jabhat al-Nusra terrorist groups in the provinces of Aleppo, Deir Ezzor and Idlib".
The strikes destroyed jihadist targets including weapons depots and command centres, "killing a large number of fighters," Moscow said.
Separately, the Syrian Observatory for Human Rights said 23 civilians were killed in Russian and Syrian strikes on rebel-held areas in Aleppo, Syria's second city.
Nine civilians were also killed in government-held areas by rebel shelling, it said.
The deployment from Iran marks a major switch in the bombing campaign the Kremlin launched in September to support Syrian leader Bashar al-Assad, as until now Moscow had only flown raids out of its bases in Syria and Russia.
Iran and Russia are the two firmest backers of the Assad regime, with Tehran commanding thousands of troops fighting for him on the ground while Russia provides airpower.
Both oppose calls for Assad to step down as a way of resolving the conflict that has killed more than 290,000 people since it erupted in March 2011.
- Benefit of Iran deployment -
Moscow has so far used short-range craft stationed at its Hmeimim airbase outside the Syrian coastal city of Latakia, as well as ships in the Caspian Sea and a submarine in the Mediterranean, to bombard Syrian territory.
By using Iran to launch long-range bomber raids rather than a base in southern Russia, Moscow can boost its firepower, military analyst Pavel Felgenhauer told AFP.
"Bombers can transport more bombs if their flight time is short," he said.
Ali Shamkhani, secretary of Iran's Supreme National Security Council, told state news agency IRNA that Moscow and Tehran "exchange capacities and facilities" in the fight against terrorism in Syria.
An unnamed military source told Interfax news agency on Monday that Russia had also sent requests to Iran and Iraq to fire cruise missiles across their airspace.
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In Washington, State Department spokesman Mark Toner described Russian warplane deployment from Iran as "unfortunate, but not surprising or unexpected."
"Frankly, that only makes more difficult what is already a very contentious and complex and difficult situation," he said.
"And it only pushes us further away from what we're all... trying to pursue, which is a credible nation-wide cessation of hostilities and a political process in Geneva that leads to a peaceful transition."
Earlier, Baghdad-based US military spokesman Colonel Chris Garver said Russian authorities had notified the US-led coalition fighting the Islamic State group in Iraq and Syria shortly before launching the bombing mission from Iran.
The coalition since last year has operated a "memorandum of understanding" with Russia, whereby the two military forces notify each other of flights during their separate bombing campaigns to avoid accidents in the skies over Syria.
- Aleppo violence -
Fighting for control of Aleppo, a former economic hub in northwestern Syria, has intensified after regime troops seized control of the last supply route into rebel-held areas in mid-July.
An AFP correspondent in eastern districts of Aleppo said there were heavy air strikes throughout Monday night and into the day on Tuesday in Tariq al-Bab and Al-Sakhur.
Men were seen pulling debris and rubble from the ground floor of a building, while others zipped corpses into black body bags.
The increased fighting has raised concerns for the estimated 1.5 million civilians still in the shattered city, including some 250,000 in rebel-held areas.
Since mid-2012, Aleppo has been split between opposition control in the east and government forces in the west, with both sides exchanging accusations of indiscriminate attacks against civilians.
The UN Commission of Inquiry on Syria said in a statement it was "gravely concerned for the safety of civilians" in Aleppo and called for "immediate attention and response" to their plight.
Human Rights Watch accused Syrian and Russian warplanes of having repeatedly used incendiary weapons against civilians in northern Syria, saying it had documented their use at least 18 times since June.
On Tuesday, US Secretary of State John Kerry discussed the situation in Aleppo with Russian counterpart Sergei Lavrov, the Russian foreign ministry said.
Defence Minister Sergei Shoigu said in comments aired Monday that Russia and the United States were close to joining forces in some form around Aleppo and "begin battling together so that there is peace on this territory."
But US State Department spokeswoman Elizabeth Trudeau refused to confirm any collaboration.