Moscow: Russia has said 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.
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.
In Washington, State Department spokesman Mark Toner described Russian warplane deployment from Iran as "unfortunate, but not surprising or unexpected."
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.
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 on 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.