MOSCOW - Russia on Wednesday dismissed a suggestion from Washington that Moscow is violating a UN Security Council resolution by using an Iranian air base for its bombing raids on Syria.

Foreign Minister Sergei Lavrov denied that Moscow could be in breach of a ban on supply or transfer of warplanes to Iran without prior approval of the United Nations Security Council. “There are no grounds to suspect Russia of breaching the resolution,” Lavrov said at a news conference in Moscow.

Russia on Tuesday began flying warplanes from an Iranian airbase in a major switch in its bombing campaign in Syria that the United States condemned as “unfortunate”. Russian forces took off from the Iranian base to carry out a fresh round of strikes on Wednesday morning. Russia has previously only flown raids out of its bases in Syria and Russia.

On Tuesday, US State Department spokesman Mark Toner said Russia’s use of the Iranian base “could very well be a violation” of a UN Security Council resolution that requires its prior approval for the supply, sale or transfer of warplanes to Iran. But Lavrov insisted that “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 ministry spokesman Igor Konashenkov suggested that Washington needed to brush up on the specifics of the resolution. He added that the US-led coalition’s bombing raids on Syria from Turkey’s Incirlik air base are not permitted by the United Nations charter.

On Wednesday morning, Russia sent Sukhoi Su-34 jets from the Hamedan base in western Iran to carry out a group aerial strike against IS targets in Deir Ezzor province, the defence ministry said, 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. Iran also defended Russia’s use of the base, with Ali-Akbar Velayati, top advisor to the Supreme Leader, telling Tasnim news agency that “the presence of Iran and Russia at the request of Syrian government is legal”.

Iran is acting “within the framework of international regulations” and “does not accept the Americans’ view,” he said.

“The only action taking place is that Russian fighters are allowed to use this base to refuel,” said Alaeddin Boroujerdi, chairman of parliament’s National Security and Foreign Policy Commission.

He stressed that this does not breach the Iranian constitution which bans foreign deployments on its soil.

Defence consultancy IHS Jane’s said Wednesday that Moscow is apparently expanding its Mozdok airbase in southern Russia, from where long-range bombers had been flying their Syria raids, with a “second runway” being constructed since May or June. 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.

Meanwhile, Rebel shelling in Syria’s battered northern city of Aleppo killed 12 people including a child on Wednesday, state media said.

State television announced “12 martyrs including a child and several wounded after terrorists shelled the Salaheddin” district in the city’s regime-controlled west. The Syrian Observatory for Human Rights said that 10 civilians including a child had been killed in the rebel shelling.

Since mid-2012, Aleppo has been roughly split between opposition control in the east and government forces in the west. An AFP correspondent in the city’s east said regime warplanes pounded rebel-held districts on Wednesday.

The bombardment came as regime forces fight rebels and militants southwest of the battleground city after opposition fighters gained ground there earlier this month, the Observatory said.

United Nations Secretary-General Ban Ki-moon has warned of an unprecedented “humanitarian catastrophe” in Syria’s Aleppo and urged Russia and the United States to quickly reach a deal on a ceasefire in the city and elsewhere in the country.

Fighting for control of Aleppo, split between its government-held west and rebel-held eastern neighborhoods, has intensified in recent weeks causing hundreds of deaths and depriving many civilians of power, water and vital supplies.

“In Aleppo we risk seeing a humanitarian catastrophe unprecedented in the over five years of bloodshed and suffering in the Syrian conflict,” Ban told the UN Security Council in his latest monthly report on aid access, seen by Reuters.

Aleppo is one of the bastions of the rebellion to oust Syrian President Bashar al-Assad, whose army is backed on the ground by Shi’ite Muslim militias from neighboring countries and from the skies by Russian air strikes.

“The fight for territory and resources is being undertaken through indiscriminate attacks on residential areas, including through the use of barrel bombs, killing hundreds of civilians, including dozens of children,” Ban said in the UN report.

“All parties to the conflict are failing to uphold their obligation to protect civilians,” he said.

Ban reiterated a UN call for at least a 48-hour humanitarian pause in fighting in Aleppo for aid deliveries and also pushed Moscow and Washington to rapidly reach a deal on a ceasefire.

Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry on Tuesday discussed securing a ceasefire, the Russian Foreign Ministry said.

Russia used Iran as a base from which to launch air strikes against Syrian militants for the first time on Tuesday. The Russian Defence Ministry said it takes great care to avoid civilian casualties in its air strikes.

The Syrian Observatory for Human Rights, a U.K.-based war monitor, said heavy air strikes on Tuesday had hit many targets in and around Aleppo and elsewhere in Syria, killing dozens.

The United States had been targeting Islamic State militants in Syria with air strikes for nearly two years.