Russia on Wednesday launched warplanes from Iran for the second time to bomb the Islamic State group in Syria, denying the action violated a UN Security Council resolution.
The strikes came after Russia on Tuesday began flying warplanes from an Iranian airbase in a major switch in its bombing campaign in Syria, prompting concern from the United States.
On Wednesday, Russian Sukhoi-34 jets took off from the Hamedan base in western Iran and carried out a group aerial strike against IS group targets in Deir Ezzor province, the defence ministry said in a statement, calling the operation a success.
The strikes with high-explosive fragmentation bombs "destroyed two command centres and large field camps for training terrorists in the area of the town of Deir Ezzor, killing more than 150 fighters including foreign mercenaries," the ministry said.
Russia had previously only flown raids out of its bases in Syria and Russia.
Foreign Minister Sergei Lavrov insisted Wednesday that using the Iranian base did not breach a UN Security Council resolution that requires its prior approval for the supply, sale or transfer of warplanes to Iran.
On Tuesday, US State Department spokesman Mark Toner said Russia's use of an Iranian base was "unfortunate" and "could very well be a violation" of the resolution.
"There are no grounds to suspect Russia of breaching the resolution," Lavrov said at a news conference in Moscow.
"In the case we are discussing now, there was neither the sale, nor supply, nor transfer of warplanes to Iran."
"These warplanes with the consent of Iran are being used by the Russian air force to participate in an anti-terrorism operation in Syria at the request of the legal Syrian authorities," he said.
"There's nothing even to discuss here."
Defence consultancy IHS Jane's said Wednesday that Moscow's use of the Mozdok airbase in southern Russia, from where long-range bombers had been flying their Syria raids, may have been disrupted by apparent expansion works.
"Flight operations may have been disrupted by the construction of a second runway" which began some time between May and June, according to satellite imagery, it said in a report.
Iran and Russia are the two staunchest backers of the regime of President Bashar al-Assad, 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.