Russia's foreign minister has rejected allegations that its use of Iranian military bases for air strikes in Syria violates United Nations sanctions on Iran.
Russia on Tuesday announced that it had launched the strikes from near the Iranian city of Hamedan and struck targets in three provinces in northern and eastern Syria.
The defence ministry on Wednesday announced a new wave of air strikes out of Iran, saying its jets took off earlier in the day from a base south west of the Iranian capital, Tehran, to strike targets in the east of Syria.
It is virtually unheard of in recent history for Iran to allow a foreign power to use one of its bases to stage attacks. Russia has also never used the territory of another country in the Middle East for its operations inside Syria, where it has been carrying out an aerial campaign in support of president Bashar Assad's government for nearly a year. Iran is also a major supporter of Assad.
Russian foreign minister Sergey Lavrov on Wednesday denied allegations by US state department spokesman Mark Toner, who the day before said Russia's operation out of Iran could violate the UN resolution that prohibits the supply, sale and transfer of combat aircraft to Iran unless approved by the Security Council.
"In the case we're discussing there has been no supply, sale or transfer of fighter jets to Iran," Mr Lavrov told a news conference. "The Russian air force uses these fighter jets with Iran's approval in order to take part in the counter-terrorism operation" in Syria.
The minister also called on the US not to "nitpick about what is happening in terms of the remaining restrictions on trade and ties with Iran".
In Syria, seven civilians were killed and nine were wounded by rocket rounds fired by armed groups on a government-controlled district of the city of Aleppo, Syria's state-run news agency said on Wednesday.
SANA reported that the rockets struck the Salaheddine residential district in the northern city, which has been divided into a rebel-held eastern part and a government-controlled western part since 2012. On Tuesday, nearly 20 civilians were reported killed in air strikes in eastern districts of Aleppo, Syria's largest city and its former commercial centre which has become the focal point of the country's ruinous civil war.
Iran allowing Russian warplanes to take off from its territory to bomb targets in Syria was an unprecedented move, underscoring the deepening co-operation between two powerhouses heavily invested in the Syrian civil war.
Ali Larijani, the speaker of Iran's parliament, insisted on Wednesday that Russia does not have a permanent military base within the Islamic Republic. His comments were geared at easing domestic concerns over the strikes. Iran's constitution, ratified after its 1979 Islamic Revolution, bars foreign militaries from having bases within the country.
In his remarks, reported by the state-run IRNA news agency, Mr Larijani did not directly discuss the strikes, though he said Iran has "co-operated with Russia, as it is our ally on regional issues, especially on Syrian issues".
Colonel Christopher Garver, a US military spokesman in Baghdad, told reporters at the Pentagon that the Russians activated a communications link with coalition officials just ahead of the first bomber mission on Tuesday.
"The Russians did notify the coalition," he said, adding that they "informed us they were coming through" airspace that could potentially put them in proximity of US and coalition aircraft in Iraq or Syria.
Asked how much advance notice the Russians gave the US, Col Garver said, "we did know in time" to maintain safety of flight. "It's not a lot of time, but it's enough" to maintain safety in the airspace over Iraq and Syria, he said.
That raises questions about whether the move was a strategic necessity or a political message from the Kremlin to Washington.
The announcement from Russia marks the first significant stationing of its troops in Iran since the Second World War.