Russian jets based in Iran on Tuesday struck targets inside Syria, the Russian defense ministry said, after Moscow deployed aircraft to an Iranian air force base to widen its campaign in Syria.

The ministry said the strikes, by Tupolev-22M3 long-range bombers and Sukhoi-34 fighter bombers, were launched from the Hamadan airbase in western Iran.

It is thought to be the first time Russia has struck targets inside Syria from Iran since it launched a bombing campaign to support Bashar al-Assad in September last year.

The United States said it was still assessing the extent of Russian-Iranian cooperation but described the new development as "unfortunate".

State Department spokesman Mark Toner said the US was looking into whether the move violated UN Security Council resolution 2231, which prohibits the supply, sale and transfer of combat aircraft to Iran.

"It's unfortunate but not surprising," Toner told reporters. "It speaks to a continuation of a pattern we've seen of Russia continuing to carry out air strikes, now with Iran's direct assistance, ... that predominantly target moderate Syrian opposition forces."

'A sizeable military presence'

Earlier on Tuesday, Russia's state-backed Rossiya 24 channel said the deployment would allow the Russian air force to cut flight times by 60 percent and increase bombing payloads.

Russian media said the Tupolev-22M3 bombers, which had already conducted many strikes in Syria from southern Russia, were too large to be accommodated at Russia's airbase inside Syria.

The Iranian airbase near Hamadan, sometimes also called Hamedan, is located in north-west Iran and the Russian bombers would have to over fly Iraq to conduct strikes in Syria.

Russian military analyst Pavel Felgenhauer called Moscow's transfer of heavy bomber planes to Iran a "major move".

"It’s not just Russian planes touching down in Iran. To establish an operational base, they’d have to move hundreds of servicemen as well. Thousands of tons of munitions, fuel, [and] other equipment to operate heavy bombers from an Iranian base. So this is actually Russia establishing a rather sizeable military presence inside Iran," he told Al Jazeera from Moscow.

Iraqi Prime Minister Haider al-Abadi said on Tuesday that Iraq, which lies between Iran and Syria, had granted Russia permission to use its air space.

Incendiary weapons

Separately on Tuesday, the New York-based Human Rights Watch accused Syrian regime forces and their Russian allies of using incendiary weapons, which burn their victims and start fires, in opposition-held civilian areas of north and north-western Syria.

"Incendiary weapons have been used at least 18 times over the past six weeks, including attacks on the opposition-held areas in the cities of Aleppo and Idlib on August 7, 2016," the rights group said.

Photographs and videos recorded by Human Rights Watch at the time of the attacks indicated there were incendiary weapon attacks on opposition-held areas in the Aleppo and Idlib provinces between June 5 and August 10.

"Countries meeting at the Convention on Conventional Weapons (CCW) in Geneva on August 29 should condemn the use of air-dropped incendiary weapons ... and press Syria and Russia to immediately stop using incendiary weapons in civilian areas," HRW said.

Fighting in Aleppo intensified in early July when regime forces captured the last supply route to the opposition-held eastern sector of the city, raising fears that it’s estimated 250,000 to 300,000 remaining residents could suffer a lengthy siege.

PHOTO CAPTION

A still image, taken from video footage and released by Russia's Defense Ministry on August 16, 2016, shows a Russian Tupolev Tu-22M3 long-range bomber based in Iran dropping off bombs at an unknown location in Syria [Reuters].

Al-Jazeera